wow....you were very lucky to get that bird down in one piece.On first appearances those cracks look very much like they were caused by vibration...that would certainly be my bet.Vibration is a killer..and it can do the job quicker than you would imagine.As anyone who has experienced the loss of a model due to flutter will tell you..it happens so fast you simply just cannot reduce speed in time to stop the airframe breaking up in flight.I know from bitter experience. However..flutter makes a LOT of noise..you probably would have heard it .
Just wondering..were the engines soft-mounted? Also..were the propellors used for the flight carefully balanced?....There may possibly have been a type of vibration caused by the fact there were two motors running in an un-synchronised / synchronised fashion...exagerated by the long booms in the P38.
Hope you can sort out the problem...that P38 is very nice model.
regards Pat