If you accept that there are moral/ethical problems with eating meat (contribution to climate change, health concerns, animals being killed and eaten, whatever), and choose to eat meat anyway, and encounter a vegan, what has to happen?
You can accept that they are making a better choice, but then you have to accept that you’re making a worse choice. Most people are cowards and protect the ego at any cost. Rather than shrugging and saying “yeah, i should eat less meat. Good for you taking the high road”, which requires accepting that you’re not being the best, you can instead grab onto any reasons why no it’s really them that sucks. That’s easier, more comfortable, and doesn’t require any painful introspection or changes.
It’s the same mechanism when people get mad at cyclists, pedestrians, people who go to the gym, people who don’t shop at Walmart, whatever. They’re doing something that makes you feel bad in comparison. Most people are terrible at that and will lash out instead of doing anything productive.
Alternatively, or maybe additionally, people are really tribal, and once they adopt the idea that vegans (or cyclists, or people driving small cars, or people wearing sandals, whatever) are in the outgroup, then they enjoy being hostile to them.
People are ego driven emotional morons. All of us. Me, too. It’s terrible.