Just a guess, but it looks like by the marks on the ground his rear dif was locked but may not of been in 4x4, so he would of gone straight up and slightly to one side before going over backwards, good thing it went to the side other wise it would of been a broke neck/back. Not that 4x4 would have made much difference, but judging by how much chain is there and the ruts that's what it looks like to me.
He had the lower draw bar holes available to connect chain to, wonder why he didn't use it. I've done that before and had the front end come up but was expecting it and was ready with the clutch to prevent a walk over.
Those pictures are the perfect poster child for that problem, almost don't need an explanation. just take one of those pics and place one of those stick figure guys that are on the safety labels all over our tractors now, in there with his stick leg broke in 2 pcs, that would be enough to scare you safe.