Starter burnt up

[Mav]

06 Pete 387 isx 870. Today I shut my truck off, the motor cut out but the starter engaged and started turning the motor. I turned the key back on and the motor fired up again. I jumped out and pulled the starter relay but it was to late I could smell the starter motor burning. I went to to TA shop and was told starter was probably bad. I have to pick a load at 02:30 going home truck has been idling fine all day. Not going to shut it off until I am home. Anyone know what caused this. Thank you