Apoptosis vs necroptosis
From my readings, it seems that when procaspases 8 or 10 are
  unavailable, the cell will shift to necroptosis. What makes these procaspases unavailable and why does a cell second the necropotosis?


This seems to be right.


  Mice with a conditional deletion of caspase-8 in the intestinal
  epithelium (Casp8ΔIEC) spontaneously developed inflammatory lesions in
  the terminal ileum and were highly susceptible to colitis. Casp8ΔIEC
  mice lacked Paneth cells and showed reduced numbers of goblet cells,
  indicating dysregulated antimicrobial immune cell functions of the
  intestinal epithelium. Casp8ΔIEC mice showed increased cell death in
  the Paneth cell area of small intestinal crypts. Epithelial cell death
  was induced by tumour necrosis factor (TNF)-α, was associated with
  increased expression of receptor-interacting protein 3 (Rip3; also
  known as Ripk3) and could be inhibited on blockade of necroptosis.
  
  Caspase-8 is a cysteine protease critically involved in regulating
  cellular apoptosis. On activation of death receptors, including
  TNF-receptor and Fas, caspase-8 is activated by limited
  autoproteolysis and the processed caspase-8 subsequently triggers the
  caspase cascade that finally leads to apoptotic cell death.
  Caspase-mediated apoptosis is important for the turnover of IECs and
  for shaping the morphology of the gastrointestinal tract 4.



2011 - Caspase-8 regulates TNF-α-induced epithelial necroptosis and terminal ileitis



  Caspase-8 has both prodeath and prosurvival roles, mediating apoptosis
  and/or preventing RIPK1-mediated necroptosis depending on cell type
  and stimulus. We found that inflammatory stimuli (LPS, lipoteichoic
  acid, or TNF-α) caused an increase in caspase-8 IETDase activity in
  primary rat microglia without inducing apoptosis. Inhibition of
  caspase-8 with either Z-VAD-fmk or IETD-fmk resulted in necrosis of
  activated microglia. Inhibition of caspases with Z-VAD-fmk did not
  kill non-activated microglia, or astrocytes and neurons in any
  condition. Necrostatin-1, a specific inhibitor of RIPK1, prevented
  microglial caspase inhibition-induced death, indicating death was by
  necroptosis.



2013 - Caspase inhibitors protect neurons by enabling selective necroptosis of inflamed microglia.



  Necrosis has long been described as a consequence of extreme
  physicochemical stress, such as heat, osmotic shock, mechanical
  stress, and freeze-thawing, which kill cells quickly and directly.
  Therefore, this cell death has been described as uncontrolled and
  accidental necrosis characterized by loss of plasmamembrane integrity
  and cellular collapse, though nuclei remain largely intact during this
  process (Krysko et al., 2008a,2008b; Vanden Berghe et al., 2010). Loss
  of membrane integrity and release of intracellular content grant
  necrotic cells the ability to induce an inflammatory response. These
  immunogenic endogenous molecules fall under the umbrella term
  ‘‘damage-associated molecular patterns’’ (DAMPs) (Garg et al., 2010;
  Kryskoet al., 2011). They include, in the case of accidental necrosis,
  HMGB1, IL-1a, uric acid, DNA fragments, mitochondrial content, and ATP
  (Tables 1and2)(Eigenbrod et al., 2008; Kono et al., 2010; Krysko et
  al., 2008a; Sauter et al., 2000). Because the nomenclature of DAMPs in
  the literature is confusing, here we define DAMPs as a family of
  molecules that are intracellular in physiological condition



2013 - Necroptosis: The Release of Damage-Associated Molecular Patterns and Its Physiological Relevance


Based on the articles above, I would say that capsase-8 is a signal that apoptosis works finely, and so the cell can kill itself whenever it wants. So if that signal is lost because of an infection, mutation or extensive cell damage, the cell has no choice, but to activate the necrotic pathway.




Figure 1 - cell survival and programmed cell death - source



  During tumorigenesis, significant loss or inactivation of lead members
  in the caspase family leads to impairing apoptosis induction, causing
  a dramatic imbalance in the growth dynamics, ultimately resulting in
  aberrant growth of human cancers. Recent exploitation of apoptosis
  pathways towards re-instating apoptosis induction via caspase
  re-activation has provided new molecular platforms for the development
  of therapeutic strategies effective against advanced prostate cancer
  as well as other solid tumors.



2012 - CASPASE CONTROL: PROTAGONISTS OF CANCER CELL APOPTOSIS



  These findings reveal that, although apoptotic/drug resistance is a
  formidable “stronghold” of cancer against chemotherapy, necroptotic
  susceptibility is an intrinsic “weak point” of cancer.



2007 - Targeting the Weak Point of Cancer by Induction of Necroptosis



  Playing a central role in cell death, caspases are targeted by
  different viruses in the aim of prolonging cell survival. For
  instance, viral IAPs (inhibitors of apoptosis) interact with and
  inhibit the processed active caspases, either by blocking the
  catalytic part of the enzymes, or through E3 ubiquitin ligase activity
  from RING domains, targeting caspases for rapid degradation via
  proteasome. Baculoviruses, asfiviruses or irridescent viruses encode
  such IAPs. Another class of caspase inhibitors frequently found among
  poxviruses include the serine proteinase inhibitors serpins:
  CrmA/SPI-2. Crma targets cysteine proteases such as host Caspase 1 and
  Caspase 8.



viralzone - Inhibition of host caspases by virus



  Pathogens specifically target both the caspase 8-dependent apoptotic
  cell death pathway and the necrotic cell death pathway that is
  dependent on receptor-interacting protein 1 (RIP1; also known as
  RIPK1) and RIP3 (also known as RIPK3). The fundamental co-regulation
  of these two cell death pathways emerged when the midgestational death
  of mice deficient in FAS-associated death domain protein (FADD) or
  caspase 8 was reversed by elimination of RIP1 or RIP3, indicating a
  far more entwined relationship than previously appreciated. Thus,
  mammals require caspase 8 activity during embryogenesis to suppress
  the kinases RIP1 and RIP3 as part of the dialogue between two distinct
  cell death processes that together fulfil reinforcing roles in the
  host defence against intracellular pathogens such as herpesviruses.
  
  The MCMV protein vICA prevents caspase 8 activation, and this
  sensitizes cells to death receptor-induced necroptosis. In addition,
  the MCMV protein vIRA blocks necroptosis and MCMV-induced programmed
  necrosis by inhibiting RHIM (RIP homotypic interaction
  motif)-dependent interactions.



2012 - Viral infection and the evolution of caspase 8-regulated apoptotic and necrotic death pathways



  One potential explanation for this is that this form of death evolved
  to provide a “backup” pathway. Indeed, cowpox virus produces the
  factor CrmA, a potent apical caspase inhibitor capable of blocking
  apoptosis (Ray et al., 1992; Gagliardini et al., 1994). Several viral
  encoded genes are similarly produced to prevent apoptosis, and thus it
  is clear that inhibition of apoptosis is an important means viruses
  have exploited to avoid immune clearance. Interestingly,
  cytomegalovirus induces RIP3-mediated necroptosis via the interferon
  regulatory factor DAI (Upton et al., 2012). This virus also produces a
  protein called viRA, a protein that disrupts assembly of RIP1/RIP3
  necrosomes and consequent necroptosis. Thus, while the physiological
  function(s) of necroptosis remains to be fully elaborated, it is clear
  that this cellular process has been around for a long time during
  evolution. It is likely that necroptosis may serve as an “Achille's
  heel” in tumor cells, and thus greater understanding of the process
  may reveal novel therapies for cancer therapeutics.



2014 - Grand challenges in cell death and survival: apoptosis vs. necroptosis


It seems like there is a 3rd pathway too, it is called ferroptosis.


  TNF-induced necroptosis depends on receptor-interacting protein-1
  kinase, mitochondrial complex I and cytosolic phospholipase A2
  activities, whereas H2O2-induced necrosis requires iron-dependent
  Fenton reactions.



2009 - Necroptosis, necrosis and secondary necrosis converge on similar cellular disintegration features



  The transition metal iron is essential for life, yet potentially toxic
  iron-catalyzed reactive oxygen species (ROS) are unavoidable in an
  oxygen-rich environment. Iron and ROS are increasingly recognized as
  important initiators and mediators of cell death in a variety of
  organisms and pathological situations. Here, we review recent
  discoveries regarding the mechanism by which iron and ROS participate
  in cell death. We describe the different roles of iron in triggering
  cell death, targets of iron-dependent ROS that mediate cell death and
  a new form of iron-dependent cell death termed ferroptosis. Recent
  advances in understanding the role of iron and ROS in cell death offer
  unexpected surprises and suggest new therapeutic avenues to treat
  cancer, organ dam
  - age and degenerative disease



2013 - The role of iron and reactive oxygen species in cell death
2012 - Heme induces programmed necrosis on macrophages through autocrine TNF and ROS production


Not just a 3rd, there are many more pathways (pyroptosis, MPT-RN, parthanatos, ferroptosis, NETosis).




Figure 2 - Signalling pathways of regulated cell death.
2014 - Regulated cell death and inflammation: an auto-amplification loop causes organ failure