Steve - an excellent interprutation of an AFCI in action!
Even without the AFCIs in the panel, 104F is a very easy ambient temperature to reach and excede in many environments. Let alone deserts, you could easily have a panel in direct or partial sunlight in a temperate area reach well above 125F. It's a metal box!
Now add some heat from heavy currents, such as AC, etc. 40C seems like an awful low testing temperature to rate electrical equipment, IMHO.
