Please read this endermen glitch.
Hi. I was just trying the latest snapshot when I decided to fight the ender dragon. I was low on health when I realised that endermen were helping me fight the enderdragon. I'm not entirely sure why but the endermen were fighting the ender dragon. I think that this is because the dragon accidentally killed one of them.
As far as I'm concerned, I don't think that it should be happening.
Its pretty funny though.