This is most noticeable in scenarios (when you’re trying to hit mobs with aoe it’s kind of obnoxious) but I’ve seen it happen with the compromised Orochi cyborgs in Kaidan too. Basically, when casting a buff on themselves mobs will sometimes (not always) run far away from the player, cast the buff and then run back. It’s kind of weird behaviour anyway but when melee mobs do it, the buff is usually expired by the time they get back into range.
Video of it happening:
You can see one mob in the middle of doing this at the start of the video and two more do it towards the end.