Also, keep in mind that the AFCI also provides GFCI protection. My guess: it is tripping due to a ground fault rather than arcing. I assume that it is not a multi-wire circuit.
I had an AFCI that tripped often. It was dedicated to the receptacles of a bedroom but one of the receptacles was switched via a switch loop to a box that housed a switch to a ceiling fan as well. Since the ceiling fan was on a seperated circuit, the shared grounds between the fan and receptacle cables caused the AFCI to trip.
