1 Because they are the minority
Humans usually consider minority groups strange as they don't fit in their system. People might make fun of minority groups just because they are different.
2 Because vegans are different
According to psychologists, people like familiar faces and objects more than unfamiliar ones. The fact that vegans are different might motivate some people to make fun of them.
3 They find their reasoning unconvincing
Some meat eaters find the reasoning used by vegans unconvincing. This could motivate some meat eaters to make fun of vegans.
4 Some vegans preach it as a cult
Some vegans try to force people to become vegans as well by preaching their beliefs as if it's a religion or a cult. This makes some meat eaters respond back with criticism. (See Why do people criticize others?)
5 Religious reasons
While it's not always stated directly, some meat eaters don't like vegans as they believe they are doing something that doesn't match their religious instructions which encourages them to eat meat.
6 Vegans try to make them feel guilty
Some vegans try to make meat eaters feel guilty for what they are doing. As a result of being pushy, some meat eaters respond back with criticism.
7 Bad history with a vegan
If a meat eater has a rough debate with a vegan then he meets another vegan, he might make fun of him just because of the stored negative emotions he has from the last debate.
8 To release stored emotions
According to psychologists, a person can make fun of another to release the negative emotions he has towards that person. If some people dislike the behavior of vegans then they might make fun of them.
9 To feel superior
According to individual psychology, people can make fun of others to reach the goal of feeling superior to them. (See Why do people strive for superiority?)
10 They believe vegans are ruining the system
Some people don't like vegans as a result of believing that they are trying to ruin a system that was established since humanity started.