That is definitely odd. Our motors sparked hard but never blew on their own, 100% of their failures were due to being stalled on the end of a locked up gearbox. I suppose if the gearbox were really resistant (debris/wrong grease?) and you were literally banging the motor back and forth at high RPM, it could blow, but even then I would expect it to just spark a lot and get hot. Not healthy for it but not fatal. You could experiment with another motor, yeah, you may have had a dodgy one all along.