Edit for clarity: I'm not asking why the Tankie/Anarchist grudge exist. I'm curious about what information sources - mentors, friends, books, TV, cultural osmosis, conveys that information to people. Where do individuals encounter this information and how does it become important to them. It's an anthropology question about a contemporary culture rather than a question about the history of leftism.
I've been thinking about this a bit lately. Newly minted Anarchists have to learn to hate Lenin and Stalin and whoever else they have a grudge against. They have to encounter some materials or teacher who teaches them "Yeah these guys, you have to hate these guys and it has to be super-personal like they kicked your dog. You have to be extremely angry about it and treat anyone who doesn't disavow them as though they're literally going to kill you."
Like there's some process of enculturation there, of being brought in to the culture of anarchism, and there's a process where anarchists learn this thing that all (most?) anarchists know and agree on.
Idk, just anthropology brain anthropologying. Cause like if someone or something didn't teach you this why would you care so much?
Probably because most western socialists come to leftist politics not through on-the-ground organizing, but rather, some dissatisfaction with their own society that leads them to seek alternatives. The first place to seek alternatives is going to be looking to the past or reading theorists, and then it depends on a lot of factors on who they'll sympathize with. It's often just aesthetics because I've organized with both Marxist and anarchist organizations and there's not a whole lot of difference when you're actually outdoors.
Also there's a long history of western states using Trotskyism as a cudgel against leftist organizing. It's cooked into the standard education now that Trotsky was supposed to be party secretary after Lenin, but Stalin betrayed them all. That's the standard understanding of how things went, so that takes a lot of time to disentangle. The primary operating mode of the western mind is an imperialist xenophobia and depending on how they get over that will say how their political outlook may shake out
Also I can't stress enough how much of simply an aesthetic it is, at least in the west. It doesn't matter. Unless you live in like...Greece or the Philippines or somewhere in rural Colombia, there aren't warring factions of Marxists and anarchists. The only sectarian conflict I've ever seen in person were when cops were involved, like the "Maoist" groups that would overturn tables at Food not Bombs or show up at PSL events to call the organizers a bunch of crackers.
On the ground you gotta understand how little of this actually matters. There simply isn't major conflict. Anarchist and Marxist groups cooperate all the time. They might have disagreements on how to do stuff or what symbols to use, but it basically never matters. The closest thing to a sectarian conflict that I can remember among the groups I roll with is around 2021 some of the anarchists I knew wanted to boycott the covid vaccine. But they ended up getting it anyway so who cares
Huey P Newton gets into this a little bit here.