It happens in many games that have fall damage, it's called "skipping to death" and it's always because of damage applied to vertical surfaces and the physics of movement.

Skipping to death happens like this:
You're running down hill. The hill isn't a single vertical surface, it's hundreds. Because of how the movement works in these games, sometimes you don't touch the horizontal surfaces (thus resetting the damage accumulation) you are technically falling the entire time, thus when you actually fall more than 1 meter in-game you finally touch a horizontal surface and all the damage is finally calculated and you just die.

Guild Wars 2 had this problem after an update, you'd just be running down a not-steep hill and die from fall damage, it was finally patched.

It seems to be pretty rare in this game, I've never died from it (been damaged by it) but I know that it can happen simply because of how the game works.