People creating missions that have, say, lots of Hirogen, for example, to get the Hirogen kill accolade.
There were initial problems with people using tricks to kill lots at once. That's been mostly resolved but it peeves some players who hunted exploration clusters for enemies that people can just drop 100 in a map.
Personally, I think if you kill them, you kill them. As long as you aren't using any tricks for easier kills, it doesn't matter. The accolades are for killing, not searching out enemies, and you could just as easily repeat missions to get the accolades now that we have replay. (The truly old school accolade hunters had to figure out how to do it without the Foundry OR replay, which meant among other things that they'd go looking for extras of some enemies on mission maps.)