According to what I read a few days ago, He put it park and climbed under the rig and it popped out of gear.
almost had this happen to me at Hale Mtn once. I had an XJ with cutting brakes on the rear, which one side had gone bad and wouldnt release after pulling that side lever. I had to get out several times to pop the bleeder on the caliper to get it to roll again. I got frustrated on an obstacle after doing a little bouncing and jumped out, chocked the rears and climbed under the back to bleed it off and continue, however I did not notice in my aggravation the tcase had jumped out of 4wd. The moment I popped the bleeder the jeep creaked and I barely rolled out from under it in time to watch it take off down the hill backward with my at the time GF in the passenger seat. It ran over a couple saplings before it found a big enough oak to stop it, she was fine, but I was freaked out bad enough to go back, load it, and head home. Lesson learned the hard way, this story kinda takes me back and hits home, coulda been me that day. Prayers for this dudes friends and family.