The only thing I can think of is the very first quest you get when waking up in Goodsprings. I did that immediately at level 2 and ended the quest in favor of the town. Maybe that has something to do with it?
Yes, if you do the Gunfight in Goodsprings in favor of Ringo and the town you will be vilified with the Power Gangers immediately. When you get those messages that pop up after you do something, make sure to read them, it tells you who likes you and who hates you, also in your PiBoy under Stats/General, it will list your standing with the factions you're interacted with so you'll know why they are or aren't shooting at you. If you are unsure of what side you want to take, just skip Goodsprings, you don't even have to talk to Sunny and just go to the NCRCF like someone else suggested. The way I go is to stay on the right side of the highway that the Goodsprings Source is located, don't cross the road and don't go near Jean's Skydiving or the Gangers on the corner lot by the trailer, just stay on the other side of the road and cross over once you get past the NCR guard and on the bridge in the town, there are 3 frag mines so be careful not to step on them or I disarm them. The convicts in the town of Primm are not Powder Gangers, they are just Escaped Convicts or Convicts, killing them has nothing to do with the Gangers. But I just go along the wall outisde the town, past the roller-coaster and up that hill to the prison, bypassing any Powder Gangers that might want to use me for target practice. This allows you to get into the camp peacefully but make sure you have 100 caps on you to pay Dawes, otherwise he won't let you in. IF you side with Joe Cobb in Goodsprings and wipe the town out including Ringo, Dawes let's you in for free and your rep with them is already accepted.
BTW the gunshots you hear if you get near Jean's Skydiving is not the Gangers shooting at you, there are a ton of Bloatflies out there they are shooting at, just don't let them spot you and stay on the other side of the street and you won't lose any rep.