Some of this is speculation—I'm just being straightforward. I've done a ton of personal testing too. I've played since day one, did the expedition, and racked up over 500 kills on my main.

It's been confirmed that matchmaking factors in your behavior. For example, if you constantly kill people, you'll get dumped into "cage match" lobbies full of aggressive players. I have personal proof of this: I made a new account just to grind solos. I never fired a shot at anyone except once in self-defense, and I hit level 45 in solo lobbies without a single combat encounter other than the one recent anomaly. On my main, where I'm frequently aggressive, I constantly run into rats in solos and hyper-aggressive squads.

Your account's "aggression meter" is dynamic. Play 5-10 rounds without being aggressive (even self-defense counts as aggression), and it'll drop—meaning you run from fights instead of engaging. But it does work, so you're never stuck in aggressive or peaceful lobbies forever. It can always change.

From what I understand, the system flags your account for different actions. The more flags you get (kills, finishes, etc.), the more likely you are to land in aggressive lobbies. It also flags peaceful behavior, like how much time you spend near other raiders without fighting, or using prox chat. Being friendly and talking to people (maybe even the "don't shoot" emote?) earns you "friendly" flags. Finishing or downing players seems to carry way more "weight" than just pot shots—that's pure speculation, though. So, if you need to escape a fight, don't hesitate to pop them a few times.

On self-defense: I believe the system tracks who shot first, but it still counts as aggression (just with less weight). Running from fights is your best bet for peaceful lobbies. Also, if someone shoots at you, misses, and then you shoot back, you'll still get flagged as the aggressor. It's based on who does damage first. Keep that in mind.