When we had a tractor that developed this problem, (AC D15) we just turned off the ignition with it in gear, let the momentum take it to a stop. In the barn hanging tobacco we would get in place and let the clutch out when we shut it down.
The 140 at one time developed a backfeed thru the alternator, turned it off, the engine would restart. With the key off, 6-8 volts was backfeeding to the coil. unplugging the white connector on the alternator solved this before turning the switch off.
What it would do is cut off, and before the engine stopped turning the current from the alt would fire a spark plug, which would turn the alt, firing more spark plugs. We figured it was getting the 6 volts from somewhere, removing the neg battery cable had no effect, unplugging the alt did.