IgA complement activation
I found some reports on it (like reference 1) but there is an oddly little amount of publications on this topic. then I found this review in Mucosal Immunology (reference 2, interesting to read) which doubts this activation. It says:


  Interaction with complement
  
  IgA lacks the residues identified in the Fc regions of IgG or IgM that
  bind to C1q, and consequently IgA does not activate the classical
  complement pathway. Although several papers have reported activation
  of the alternate pathway by heat-aggregated, denatured, or
  recombinantly generated IgA, this seems to be essentially artifactual,
  and intact native IgA antibodies complexed with antigen inhibit
  complement activation induced by IgG or IgM antibodies. This
  effect is also replicated by Fabα fragments generated by cleavage of
  IgA1 antibodies with IgA1 protease. It is telling that mixed
  aggregates of heat-denatured IgG and IgA activate the alternate
  pathway in proportion to the content of IgG, and that C3b becomes
  covalently linked to the IgG heavy chains, not to IgA. Intriguing
  reports that IgA antibodies promote complement-dependent lysis or
  opsonization of encapsulated bacteria probably also arise from
  facilitation of alternate pathway activation by bacterial
  polysaccharides


It names three papers to underline this (which are number 45-47 in the reference list of the article), which can be found as references 3-5. So the question here is not only how the mechanism looks like, but also if this is real or an artefact.

References:


Activation of complement by human serum IgA, secretory IgA and IgA1
fragments.
Structure and function relationships in IgA
Anti-inflammatory activity of human IgA antibodies and their Fabα
fragments: inhibition of IgG-mediated complement activation
IgA blocks IgM and IgG-initiated immune lysis by separate molecular
mechanisms.
Activity of human IgG and IgA subclasses in immune defense against
Neisseria meningitidis serogroup B.