I have set up Red Rocket as my Companion's settlement. All of the recruitable characters (except Strong and Curie) currently reside there. I was making a pit stop at Sanctuary to gather some junk for building a 3rd ground water pump thingy, and when I ran back, I saw a vertibird flying overhead. My 5 turrets surrounding the station and at least one of my companions were firing on it. 2 turrets were damaged, but the vertibird was blown up. And a BoS corpse was found. I'm not at a point in the main quest where

, although I did .

in that case yes they can attack you. Just make sure everyone has decent armor and weapons.

Is it actually possible to make BoS an enemy without any quests, but rather by shooting their flyers for example?

Well you have to get into the main quest as far as "family reunion" (not sure if spoiler) and the BOS arrive in force.

I know that. They even attacked Nick in the streets (or he hit their sergeant by accident and BoS troopers returned fire), I had to exterminate them to help Valentine. They didn't become hostile toward me, though. Hence I wonder if it's possible to have BoS as the enemy without advancing further down the questline and

All it takes is a stray bullet, and the BoS is an enemy.

Once the BoS' airship enters the game, you'll see random fights throughout the Commonwealth. I was attacking an area with super mutants and all of a sudden, the BoS flew in, dropped a few, and started fighting them. Unfortunately, as I was firing, one of my shots hit a BoS member and as soon as they were done with the mutants, they turned their guns on me.

It's bad enough one of these idiotic whirly birds crashed on top of me, now I have to watch these idiots when they're around. Worse part is: they took out a Legendary I had down to 2% health, negating any reward.

I'm pretty sick of the BoS. They're my enemy. Jerks.

Hard to say. Try running a few hits on their frequent airdrops and see if you can't insight some bad blood.

I'm just wondering if one of my companions is always hostile towards any BoS members (even if you are not) and if they incited the attack.

protip: if you shoot the leader of the BoS dead and jump from the airship, they will follow you and they will kill you lol