I had that problem in the Mojave Outpost and Camp McCarran. No other NCR locations had that problem at all.

In both of those locations, the sniper would shoot at me in the former and the questgiver for Three Card Bounty would shoot at me in the latter. I fixed it by inputting console commands to get them to stop attacking, otherwise if I come too close, every NCR trooper in the vicinity would randomly fight me.

Why has this happened? I'm wearing a Vault 101 jumpsuit (got it from TTW) and I tried dismissing my companions with no effect to the game.