Cardiac pacemaker: how can different nodes have different frequencies?
The AV node and any other nodes are suppressed by the SA node because it is the fastest.  If the SA node is sick or slow or there is a conduction problem one of these other nodes can take over calling the beat. 

http://www.cvphysiology.com/Arrhythmias/A018. bold emphasis mine. 


  The higher frequency of SA nodal firing suppresses other pacemaker
  sites by a mechanism called overdrive suppression. If a latent
  pacemaker is being depolarized at a higher frequency than its
  intrinsic rate of automaticity by an adjacent cell that is driven by
  the primary pacemaker, then the increased frequency of depolarizations
  leads to an increase in intracellular sodium ions because more sodium
  ions enter the cell per unit time. This increased sodium stimulates the Na+-K+-ATPase (increases its activity) to expel more sodium from the cell in exchange for potassium (see figure). Because this pump is electrogenic, increased pump activity increases the amount of hyperpolarizing currents generated by the pump. This drives the membrane potential more negative, thereby offsetting the depolarizing pacemaker currents (If) being carried into the cell. This effectively prevents the pacemaker currents from depolarizing the cell to its threshold potential, and thereby prevents the spontaneous generation of action potentials. If the cell ceases to be driven by the SA node (e.g., because of AV block), then the additional hyperpolarizing currents will be lost and spontaneous depolarization and action potential generation can occur.