I mean, because boarding first or very early is clearly very important to them, and as a result they chose to spend their own time queuing to board instead of sitting down comfortably in a chair.
Sure, that attitude is a bit on the nose, but if you like to board early, because for *whatever* reason it's important to you, then seeing someone walk to the front of the queue near the end of the boarding process, and thus see a potential reason to have that taken away from you, would understandably make you uneasy. Taking it out on someone like that is pretty poor, but if they looked you over to see if you were just asking a question of the gate staff or actually intended to cut in, I can completely understand.

They invested their own time on something important to them, and they don't want to see it taken away by someone who didn't make the same investment.

I'm not excusing their behaviour in the instance you describe, but it's straightforward to explain.