they attack from behind, at least in the fatalities I've read about
That is correct, mountain lions usually attack from above or behind in order to get a clear path to the base of the skull, where they bite in order to kill their prey quickly. Mountain lions are not particularly rugged, and can get injured in a fight, so they will break off the attack if it looks like the prey will struggle for very long. We had a mountain lion attack near here a few years ago, the cat attacked a school kid from behind, knocked him down, and bit the backpack he was wearing when it flipped up over his head. Fortunately, when the child kept screaming and thrashing about, the lion left. That kid may never know how lucky he is . . .