I've had 4 mysterious massive trauma events just like what was described by OP. Stairs have always been problem, but this isn't that. Twice I was running through the woods - screen turns red, player screams in pain, and shirt or pants go from pristine to ruined. It usually results in leg fractures too.
The last time it happened, I fixed myself up and stood there waiting to see if it was a sniper (I was doubtful), and nothing happened.
I think this is a bug that was introduced in .45 Stable. (I've got 600 hours in since SA launch).