lots of things can cause this issue were you jumping curbs? Hot dogging the mower i.e. popping wheelies ? if so you caster wheel brackets are bent.
have you properly greased the joints for the deck so that nothing is sticking?
lastly did you put all the hardware back on the bolts in the same manor you took them off when you changed blades? are the blades on right?
Do it better than the last Guy!