So I'm going to copy a bit from their comment:

You automatically assume veganism a position of superiority and everyone who’s annoyed is annoyed cuz they’re inferior and refuse to do self work.

This is the key detail, though it is not YOU who are assuming anything, or being "holier than thou". The only thing you're doing is sacrificing something to "do good".

Problem is, for people like /u/LowriderMoogly, their conscience has noticed that last part, and is judging themself for it, thus creating a feeling of discontent; Their conscience is telling them that they too ought to be better.

In this sense, the very existence of vegans/vegetarians/etc is a constant judgement that one should've been and should be better than they currently are.

However, rather than correctly identifying this as an internal feeling, it often gets projected onto the cause; This is aided by the relatively few vegans that are smug assholes about it.

And it's quite trivial to see this being the case; People get mad not at the smug assholes, but at all vegans, at all cyclists. If you check the video carefully you'll notice that the truck's breaking some traffic rules as well here. (Yes, cycling on a highway is stupid. Doesn't mean the truck gets to break the rules as well.)

Okay, the above is just a more verbose version of what BorisYeltsin09 replied to the same comment you replied to. To expand and actually answer your question:

I like animals and don't want them to suffer, why's it okay to mock me for that?

Because most people feel the same about vegetarians. That's all that makes it socially okay.

The "cyclists are assholes" attitude has the same cause; People's conscience judging themselves after seeing others make sacrifices for the environment. But crucially, it isn't really acceptable in countries with a lot of cycling. Saying "All cyclists are smug assholes" in the Netherlands is going to get you a lot of flak.

In this particular sense as well, /u/Radioactive-235 is entirely correct; Cyclists are the vegans of traffic/transport.

Fortunately, this also suggests that as veganism/vegetarianism/etc become more common, it'll become way less acceptable to flame at them.