Riding a two-wheeled contraption, by pedal-power alone, with the stresses imparted on the bottom bracket housing the pedals' connection to the rest of the drive train, imparts some major stresses into the frame where said BB is joined to the other bits.

'Cowboy kickoffs' not withstanding, where a BB connects to the rest of the frame. has to be the most significantly stressed part of a bicycle's frame construction.

I'm not surprised the OP's frames suffer cracks at those joins on a repeated basis... he's on the far right of the Bell Curve after all, a machine among mere mortals giving frame engineers nightmares.