It definitely has been happening often and it is really frustrating. Recently, I shot it with my gun a few times after the first stinger missed and the second one hit it. As far as finding a way to do this on purpose would be fun but quite devastating for the game. You can avoid everything with coldblooded and keep playing.
It might just be that, sometime stinger missiles/other missiles are bent. They don't always hit their target. I don't think that's what happened here, he fired off way too many missiles for it to that. It was some sort of glitch...