Why would a riding lawnmower stall when the deck is lowered?

More than likely your safety switch on the seat is not making connection. There is a plug under the seat. If you are lucky it just came unplugged. If it's not unplugged, you can bypass it to see if this is the problem. Just put a jumper wire from one side of the plug to the other. If it works then I recommend that you replace it. This is an important safety feature.