bunk exceeder, the guy involved would have had a key to the throttle lock as he would have been authorised.
There's no 100% surefire way to stop this sort of incident occurring.
Forgive my ignorance, but surely there was no flight plan filed and that specific aircraft was not due to depart, so at some point an unauthorised plane was taxiing around SEA and it wasn't noticed.
Surely had someone realised that plane wasn't supposed to be moving at that time then it could have been intercepted a lot earlier.