Maybe your wearing faction armor or you have done something to piss them off. Try leaving and waiting for three days then coming back.
It's a guy who attacks people who have done crimes against the NCR. But there is a bug where he will attack you for minor things like stealing or hitting a guy in the balls with a shovel
Try approaching the Embassy with a NCR disguise, and if you get attacked, lure the atckers away from sight and shoot'em down