Neutrals from two circuits tied together or a neutral faulted to ground. Arc fault breakers also trip on a ground fault.
Removing the neutral from the AFI and landing it directly to the bus bar will trip out the unit on ground fault if any current at all passes through the hot conductor, just like if you'd done it to a GFCI. This method can't be used to troubleshoot.
If the AFCI only trips when the light switch is turned on, even if there are no bulbs in it, I suspect a wiring fault with the hot conductor, either a crossed neutral connection or a short circuit somewhere. Nail through NM will do this. At least you can narrow it down to that lighting string!
Either way, it's been a month, did you ever find the cause?