Tribalism is the death of collaboration.

As soon as someone's threatening your tribe, or even speaking against them, there's a natural instinct to protect your comrades. You can't just talk it out like rational people if someone sounds like they're going to attack your right to exist as you think you need to be able to exist. I think this is at the root of some bizarre behavior by certain tribal groupings, including but certainly not limited to vegan people.

I suspect also that people who like to cause disruption in the community have seized on this as a very effective tactic for sowing division, by creating a tribal conflict that there's no possible win-win solution to or easy way to disarm. The vegan cat food thing is a perfect example. I suspect that this particular instance is another of those, either just a misguided vegan who is stirring up excitement and visibility for their cause, or else someone very deliberately trying to stir up excitement and conflict for non-vegan reasons of their own. Whether you agree or disagree with them, there will be reasonable people who will have reasons to get pretty animated about disagreeing back with you. And so... conflict.