Definitely normal. I think these probably occur in one of 20 to 30 games. They will only occur inside of a rift.
If you can kill a significant amount of them then it is usually a lot of fun (screenshot will come when I get back to the computer it is on). If you are playing above your level it can be discouraging to only kill one or two.
From what I have seen there does not seem to be a pattern to these groups forming with regards to people, levels, the type of game, etc.
Screenshot as promised
And video just for fun, this was in a t13 rift