Transmission fluid leaking?

Do not put it in reverse!

Or, all automatic transmissions pump fluid to a cooler either inside or in front of the radiator. It is not unheard of for one of the metal lines to pop a leak due to rust. The pressure the fluid is pumped up to is a real lot higher in reverse than it is for the forward gears, so if a line has a small leak, it will leak much faster in reverse. Start by looking underneath to see where the puddle is forming. If it is under the engine area, meaning ahead of the doors, it is most likely a cooler line.