If these are now catching fire without an external ignition source, it's not good
True, but we don't know that to be the case yet, as Lonewolf_50 pointed out, folks will be looking for possible sneak circuits that could have caused the problem. If I have correctly identified the ELT on Honeywell's site there are a number of remote connections available so, although it should be unlikely, perhaps there is a back-feed from the airplane system that caused the issue? It is very bad luck for Boeing that with over 3000 of these ELTs out there, the very first one to combust was one of the 50 or so in a 787!
Edit to add link:
http://www51.honeywell.com/aero/comm...06_AFN_ELT.pdf