The only thing that I can think of is that you fused the contacts on your start switch when your old rectifier short circuited and tripped the breaker. This would cause the mower to remain in the On state, such that it would start running immediately when you plug the cord into a live AC outlet. Trace the wires from the start switch and check them for a closed circuit (zero Ohms). If so, then you will need to replace your start switch assembly.