JD 316 Cranking on it's own...

So the other day my family and I were working on having a new well put in and I had my new to me 316 out helping with spooling wire for the pump. The tractor was just sitting there minding its own business when all of a sudden it started turning over with nobody near it. It didn't start but kept cranking until I pulled the negative cable off the battery.

So far I have checked the battery cables for chafing and have not found anything.

I have disconnected the ignition switch and it still does it. I am pretty sure that only leaves the solenoid left to be at fault am I wrong?

Thanks for any help.
Maineiac