Well, disclamer it's all hazy, but iirc the fact that he killed the conversation.
So at the back of the class we were talking, like 3-4 of us at least, and one guy would constantly rock up and add nothing to the conversation. There were times where he asked me aside (like to talk in private) but had no topic (just looked at me all gay-like, what?), so I just told him to piss off and went back to the group. After a while we didn't even want to hear him out, the moment he speaks our train-of-tought dies. Later it became akin to bullying.
There were more to this than that but there always is. If I have to give advice a "what are y'all talking about" so you can join in, instead of a "wow nice shirt" or some conversation starter when we are already talking about something and that something is completely irrelevant of shirts.
But ye, imagine the scenario where you are talking about how nice it was that no annoying guy were at the party yesterday, and said annoying guy joined in going "nice weather, so what were you doing yesterday?". An extreme example, but it's a bad idea to go to a group with an ice breaker when there is no ice to break. My two cents is that asking about the topic is nice caz if they don't want you to know or to talk to you they'll just go "not much" or something similar and you can instantly read that you are unwanted and leave instead of adding more fuel to the fire.
The brighter the better?