For me, I think it mainly has to do with callousness, self-importance, aggression and boundary violations. When they violate what I think should be boundaries other people should respect of others, but instead they go right ahead, especially if they seem to be aware and intentionally doing it.
And those boundaries can be merely social.
For example, when someone assumes they can move into someone else’s personal space, address people aggressively without reason, or ask personal questions or offer personal advice without permission, or tell people what to do, even if it would reasonably make the other person uncomfortable, I tend to find that creepy.