I think it's an instinctive reaction we developed to catch ourselves when we are falling from a walking position. Sure, you can injure yourself catching yourself from a walking fall that way, but probably less severely than not catching yourself at all. I haven't looked into this, but it seems to me that the overwhelming physical impulse is to do anything that prevents your head from slamming into the ground. The added speed of a moving cycle is just too much energy to catch through the outstretched arms, hands and shoulders without likelihood of serious injury.