Those cube-type self-resetting breakers can get more sensitive as they age, and also if the wire connections to them are not good and tight. They're simple thermal devices--pretty much just a bimetallic strip between the contacts--and any poor connection or extra resistance causes it to heat up more and open sooner. I think the manual reset versions are similar in how they are tripped, but I've never taken one apart to see for sure.
I don't think the solar controller has anything to do with the main house breaker tripping; neither would the engine alternator, unless somehow the current from it goes through the breaker (which would be most unusual).
You might want to double-check that your solar controller won't be (or wasn't) harmed by having the solar panels connected but the battery disconnected; some can fail under those conditions.
