first let me say I am an electrician, and satellite tech. Second I do not know all the facts of the case, so all this is theory
The first thing that bothers me is anyone that keeps reseting a breaker that trips they trip for a reason in this case I'm guessing a short, on a general lighting circuit I doubt that an IRD would trip since they use Milliamps of electricity, though it is possible that the box had internal problems. This could have been easily troubleshooted by simply unplugging the box, and testing the circuit. We don't know if he did that because the article doesn't tell us that.
It is possible that he knicked, or drilled through a wire since fires are caused by arcing then this fits what we know. It is possible that it could have ignited the truses, or insulation in the attic. This is all assuming he knicked a wire.
It is possible that had had issues there before, Did the installer even go in the attic? Again we don't know. He could have stepped on a wire that was stapled too tight, and the romex shorted. At this point it is a grey area that could put the inspector, and the electrician liable. Unless it is over 5 years, which puts it on the homeowner, or maybe the installer lol!
As far as the transformer, it sure can cause a fire. I doesn't matter if it is low voltage transformers of any kind give off heat, and if the connections are loose they will arc, and can set insulation ect on fire.
We don't have very much info, just what the homeowner, and insurance company said. Just my 2 cents