I recently watched a clip on Discovery Channel, where I saw deers crossing a river full of crocodiles, ignoring the fact that some of them would have been killed doing so.
What could be an evolutionary reason for this behaviour?
The reason for their behavior is that exactly the fact that even though some of them do indeed get killed, most of them survive -- what you have said. The deer cross the river because that allows for the better continuation of the species rather than if they had stayed on the original side of the river. You could think of it this way: If there was a crowd of 5000 people crossing a field of grass and aliens were abducting them one by one, the actual chances of them abducting you are rather little.