Could be that it hit the AC line too, and that if it did, the ground and AC line could still be at vastly different potentials (enough to arc across the insulation from hot to neutral at the outlet) even though lightning struck both simultaneously. But, I think we can all agree the earthed house did not "ground out" the lightning bolt. If it did just strike the structure, I would expect the effect I described. Draw out a grounded house with the panel connections including local ground rod, the connections going to the pole or remote poco transformer, and the grounding of the neutral at the transformer, and I think you will see the same thing. The 120 volt power is essentially at ground potential - its only 120 volts away from what is considered 0 volts at the pole and everywhere except from where the strike hit. The 120 volt lines will rise or fall with the voltage of the POCO neutral they are tied to, rather than the ground potential at the customers end.
