A defective reverse position sensor, or a shorted wire that leads to the sensor.
The condition is mimicking what happens when you select reverse, and using the Mow in Reverse over rule function permits the PTO to stay engaged.
Inspect the wiring leading to the reverse position sensor to see if the wire has chafed and worn through the insulation.

To add.

If you are mowing over rough bumpy terrain when it happens. Check to see if you can adjust the seat interlock safety switch. A light weight person can easily cause the seat switch to trip the PTO shutting off the blades.

I think that Cub Cadet designed their seat switch for a 400 lb person..lol

OP - read your manual. There is a ignition switch position which engages the "REVTECH" feature on that machine that allows you to mow in reverse without the PTO disengaging....if the switch is left in normal, every time you depress the reverse travel pedal, the PTO will cut off...