Is this a normal fault mode for an arc fault or what is happening in my wiring to cause my AFCI to trip
For everyone's info, and because it could help someone else, here is the actual answer in my case.

While I had connected the white wire coming out of the AFCI to the ground bar, I hadn't connected the neutral from the circuit to the AFCI.   In my case it was because I didn't realize the extra screw terminal was actually there and the documentation wasn't helpful.

Bottom line:   If you install a combination AFCI and connect the hot but not the neutral, it will not pop until there is a load.   After the fact, that actually makes sense.   This is like a ground fault from the AFCI's point of view because it can't see the current going back through its neutral port.