I was playing in my main survival world, which is actually dual survival with a friend. Therefore, to clarify, this was a server I was playing on (multiplayer), privately for me and my friend. He went exploring for the new rabbits in the latest snapshot, and there just happened to be a field full of rabbits, which also included at least 7 killer rabbits! Now, I am aware that this may be a case of extreme luck, but I would think the chances of one of these killer rabbits spawning, 1/1000, would be close to impossible for 7 of them spawning all in the same area.