We did a lot of investigating and found several clarifications that I didn't have correct in my previous post. New summary of the problem:
It's a Siemens Type QAF 20 amp breaker, 2006-built home - the circuit has been labeled "beth plugs." We isolated the outlets that this controls, and they are several 2nd-Floor outlets as well as our TV/Cable box outlet on Floor 1.
There have been no trip incidents for the 3 years we've been here, and we have not added any new draws to that circuit.
The first time it tripped was Monday. We were watching TV while installers were putting radiant barrier up in the attic. They said they hadn't plugged anything in.
When this breaker tripped, I found that the GFCI outlet on the counter in the kitchen had also tripped. It is on its own separate "kitchen plugs" circuit, however, I was unable to reset the tripped "beth plugs" breaker until I reset the GFCI outlet in the kitchen.
This kitchen GFCI outlet and its associates are on the other side of the wall from the downstairs TV outlet controlled by the tripped AFCI breaker. It appears that both are served by wires dropped down inside that wall from 2nd floor crawl space.
Yesterday, my wife was at home watching TV (installers/equipment gone as job was finished Monday) and it happened again. Same outlet tripped in kitchen, same breaker tripped in garage.
What could be going on here? With no new equipment adding draw and the GFCI tripping as well? Some have suggested faulty GFCI receptacle tripping the nearby AFCI breaker, even though they aren't same circuit. Others have suggested faulty breaker. And others have said installers probably messed up a connection somewhere. Any clues?
I can't make the breaker trip for experimentation, which makes it very difficult to investigate. We went through the house using one of those wiring testers and all showed as good.
Thanks for any help at all.