During residential subpanel replacement installed CAFCI on kitchen light circuit. Two weeks later, breaker trips and won't reset. I isolated segments of circuit until breaker would reset and narrowed fault down to a short switch leg. Peeling back insulation revealed the rat-nibbled romex in attached pic. Presumably the CAFCI is doing its job. But nothing else has changed in the two weeks, not even a light bulb change. The rat work is old (rats exterminated a year ago). Trip happened shortly after it started raining (and foggy.) Location is not wet, but it is within inches of attic vent and basically exposed to outside air. Could it be the increased humidity?