It isn't that you like something, it's that you don't want to be associated to some other things. And you don't want to be associated to those other things because of some pretense about what associating to those things means.
Maybe this is confusing because of the phrase "false pretense", which is redundant and can lead to some people thinking that there's such a thing as a pretense that isn't false-ish.
At the end of the day, I think the pretentiousness comes from superiority signalling. When a vegan talks about how eating meat is wrong the implication is that they feel morally superior to those who eat meat. And when someone rejects things generally, as opposed to just liking their own things, it implies that those things they reject aren't worth consideration, and hence anyone who does has some inferior judgement or taste.
The statement "I would never listen to disco music" implies there is something wrong with doing so, and hence something wrong with those who choose to. "I would never listen to disco music" is a pretentious statement.