A riding lawn mower can move in either forward or reverse direction when in neutral and it can become quite annoying. Creeping is common when the drive linkage is out of adjustment or the transmission has a problem.
The drive linkage on many riding lawn mowers is pretty flimsy in their drive to reduce the cost of the mower. This is the first area to inspect to make an adjustment and check for any debris that may be causing the linkage to bind up.
Sometimes mud will build up in the shifting rod on the transmission, so check for that and a buildup of grass on it as well. Also check to be sure that the linkage is not bound up by rust and will move in its full travel.
Make any adjustments that are necessary and then try to operate the mower normally and see if it still creeps. If it does, it is probably a good time to call in some professional help to get the transmission figured out.
You can find a riding lawn mower transmission repair specialist on our Locations page.