Possibly a defective, or loose, seat switch.
The deck engagement safety interlock switch on this Deere makes the final decision of whether it is safe to permit the blades to become powered.
With a defective, or loose seat switch, the logic is that the seat is unoccupied and therefore the engine must be shut off.

Thanks mownie

You were right on - the switch was falling off
It was an easy fix (with your help)

Glad I was of some help and thank you for the feedback.