Sup, Forum!

So I just went back to my Lucky 38 suite and ED-E was "dead". I play on hardcoe so followers can die but I'm pretty sure ED-E would be dead anyway because he wasn't my SET follower while I was gone. I came back with Boone to get him for my travels but he was just there, murdered in cold blood, on the floor of my bedroom... I can remember that certain followers will attack eachother when left alone in the lucky 38 suite, but which ones are they? I can only remember cass fighting with someone during my last playthrough(2012) but I can't remember with whom and who else starts fighting with eachother. So could you guys help me out a bit? My memory seems to be a bit dusty when it comes to this topic..