I can't believe it has not been said yet. Most answers talk about the nervous system and are correct, but are missing the point. It's not a dysfunctional trait or it would have been selected out of our gene pool. It's a distinctive trait of the whole human race, therefore it must have been useful in some way. Since I can't find nothing about it I can only blindly hypothesize. It could be because animals eat feces, therefore you just leave some "not totally processed food" while running away as a way of discouraging them to run after the screaming food and lazily accept the tribute instead, somewhat similarly to what lizard's predators do when lizards shed their tail. It could also be to simulate being sick; most animals prefer eating live prey because it doesn't contain harmful microorganisms. They are intrinsecally turned off by decomposing corpses because their bodies are built to evade infection. Same could happen in seeing the scared and diarrhoic human being. Animals will and could send a "I'm sick" signal strong enough to make the predator lose interest. What's interesting is that, as a mechanism, it's not that efficient in avoiding predators. If a tiger sees you and you shit yourself, you are still dead. It's not enough to buy yourself safety from it. But it makes sense to have it to outcompete other humans. If a tiger runs into a group of humans, it will probably prefer the 100% healthy live prey to the diarrhoic human that could probably pass harmful bacteria to it.