If someone is actually putting the master switch on without the APU running, then I'm guessing they want to get a leg up on a possible APU start after takeoff (engine failure, generator drops off line, etc) since they won't have to wait for the ram air door to cycle open
I doubt if this could be the reason. EFATO is not such a deadly emergency that you require APU in a split second. ECAM or STATUS don't even mention APU start. Starting of APU is only mentioned in FCOM Abnormal procedure There cannot be procedures for phobias.