The only thing I can think of is that marauders are on some kind of timer? I was on a Cro tank with an Arcanist DPS partner, had a marauder spawn and the DPS died; I had just colo'd the guy so i kited around the area so the AOEs were away from his corpse while I built ult for a necro rez; got the DPS up and we started to fight the marauder, but a few seconds later he just disappeared, along with all the other mobs that were up at the time, the area reset and we lost a thread, even though we were both alive... and it wasn't like we both got oneshot or something, we were both still alive.