Some monsters that spawn when you bring a large party are hostile to the smaller monsters which means that when your party gets there, a lot of them are killed, eaten or injured before the fight even starts..

The Big Ones are hostile against the small Ones I think..

Don't know its a similar bug or not, but in the hoarran quest, half the undead joined our team and fought the other undead.

Sounds kinda like this one though.

Well, some evil clerics can turn the undead and they become their own.

I'm aware of that, yes.

But this was over 10, and I don't think our party had clerics.

Our party had no clerics.

It was a simple matter of half the undead being "blue" instead of red, from the very start. Which was, I admit, quite fun to watch.

On the matter of the OP, I can confirm that this happens, but I don't know if it's a faction bug or just an area-of-affect attack harming the Smaller Enemies since whenever this happened they always were dead before we arrived there.