Here is something else to look for. I am a licensed Electrician of twenty years. I encountered something very similar. The backwired type of devices only require less than a 1/2' of insulation to be stripped from the romex. If more than that is stripped the wire will not completely seat in devices. This goes for switches and receptacles. In a new housing complex about 4 weeks after turning on and testing the circuits we started seeing breakers tripping on circuits that were previously working as designed. What I found was the bare ground wire was laying across and touching the exposed copper of the Neutral conductor because too much insulation had been removed during installation. Even thought the "N" conductor and the grounds are landed electrically at the same point in your panel the Neutral imbalance is enough to trip the Arc Fault breakers immediately.
You should call an licensed electrician out to test the system. They should be able to test and find the fault fairly quickly and you will be money ahead. Remember time is money.