Looked to me like it had been wounded and was having trouble moving. Either that or the hill was so steep that it was moving kind of slow, but I've hunted hogs in the California foothills and know that a steep hill doesn't slow them down.
Since the video was alread on it, I think they thought it was dead and down. There surprise seems pretty apparent by the video and the guy rolling down the hill with the hog!!!
Truly amazing they didn't shoot each other.
Thanks for the link, it was fun to watch.
Eddie