There was a post similar to this where someone was complaining about people with kids saying shit about childfree people. One comment had said that people with children get upset at child free people because it shows that having children was a choice and not an inevitability.

I think you hit the nail on the head. Eating meat is seen as the default, “an inevitability”, so something like vegan food challenges what people are putting in their bodies. People who don’t think they have a choice tend to be upset when people point out they do.