I'm guessing it has to do with damaging enemies, but not killing them. Someone later kills them and you get your share of the XP or loot if it was related to an event. I have gotten decoy pursuits when I do not even have decoy unlocked because I killed an enemy shooting at someone else's decoy.

Actually, I went AFC (Away From Controller) to cook myself and omelet, but I was near the academy and I think I was unintentionally getting credit for the event that takes place there from time to time because when I turned my controller back on stuff started rolling in.