I think the seat switch is good place to start. Are you certain the switch is being depressed?
If there were problems with the hydro, you may see the service light blinking on the right fender.. It blinks in a pattern that indicates certain problems depending on what is wrong with the transmission. Also, is there any chance that some wires under the tractor near the hydro pedals were pulled on while you were bushhogging? If these connections become loose or come apart, the tractor may not move at all. (not sure if it would stall though)
I think you can test the seat switch by sitting in the seat, and pulling up on the PTO switch and see if it comes on. (you don't need an implement on to try this). If the PTO comes on, stand up and see if it kicks it off.
Have you been able to start the tractor at first and drive it cold consistently? Then, once it warms up it acts up? Or is this more of a problem all the time now?
One more place to look, is there any chance the brake is stuck on? Some of these linkages stuck in the on position, and need a good swift kick in the pedal to get them unstuck.
Let us know what you find out.
good luck