When you are crouching you are able to move down by half blocks so you can move down stairs or half slabs and so on. Well you can also fall off of blocks in this way by moving in a direction off of the block while falling. For example, on the end boat, at the back there are upside down and right side up stairs. Well if you are moving forward and right at the same time down one you will fall off. So you call fall off of the blocks while crouching.