It's NOT you. Well, it is, but it isn't.
It took me many years to figure this one out. When people are having a conversation in the workplace, loud enough so everyone in the room can hear it, it IS an open conversation, which means anyone can join in.
If they're acting like you are butting in, it isn't you butting in. It's them being uncomfortable with you. There is something about you, your demeanor, and your conversational style they don't like. In effect, they're being exclusionary. They're being rude.
I couldn't tell you what it is about you that makes them uncomfortable other than that you probably don't have enough practice diving into conversations like that to be very adept at it. There is only one solution to that problem I've ever found that works, but it takes guts. Even if you feel like it's butting in, if it's an open conversation, do it anyway. Learn to ignore awkward reactions. It's not easy, but that's how I learned. It was awkward at first, but over time, with many different people, I slowly learned to become better at conversing. Eventually I became good enough at it that discomfort in others is something I rarely see anymore.
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I'm not likely to be around much longer. As before when I first signed up here years ago, I'm finding that after a long hiatus, and after only a few days back on here, I'm spending way too much time here again already. So I'm requesting my account be locked, banned or whatever. It's just time. Until then, well, I dunno...