What would cause the pulley on power steering pump to break

First check alignment of new pulley/pump. However I don't think this is the cause, I think a seized pump is causing your pulley to break free. Before you replace a power steering pump that failed, you should flush the system, so metal fragments can't get into the new pump and also cause it to fail. If the new pumps fail and seize up then it may appear it was the pulley that failed. If in fact the pump seized and caused pulley to break free, you will need to inspect the belt and possibly replace it too. Then flush the power steering system or just replace pump and steering box/rack, although a rack can get pricey.