I had 2 button heads going at it for 30 to 40 minutes at a time on numerous occasions this year on cell cam. Deer chasing each other through the woods back and forth. They act like pups at times. Good entertainment.
Little guy might have taken a solid hit to the noggin the night before. I've seen bucks trying to shake the cobwebs and fight the last fight over for days after a solid hit. The spike's balance appears pretty good, so if he has a concussion it is mild at worst!
Or, more likely, you are right and he is shadow boxing.