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?
I think AssortedBiscuits answered your question in the first couple sentences of their comment:
It's really not any deeper than that. There's no need or reason to single out anarchists from any other average westerner when analyzing the source of animosity for the USSR because the answer is going to be the same whether you're talking about chuds, liberals, or anarchists. Even the non-western anarchists who hold a grudge against the USSR, the answer is probably still the same just because of the prevalence of western cultural hegemony all over the world. In your edit, you specify:
But the answer to that is the same information sources you yourself were probably exposed to early on. It's all the same shit we're steeped in, the ubiquity of anti-communism throughout western culture. Animal Farm and 1984 were required reading for me in junior high and high school respectively. The class discussions around these books were centered around teaching us that the USSR was corrupt, oppressive, and that these communist ideals that may sound like good ideas will always and invariably lead to "authoritarianism" and "totalitarian dictatorships" like the Soviet Union. Everyone absorbs that shit young, even the people who might later go on to question the truth of what they were taught, like anarchists.
You say
But no they don't. Not as newly-minted anarchists anyway. That brainworm software was already installed long ago before they became anarchists. A major part of becoming a leftist is going through a process of uninstalling all that brainworm malware. Anarchists who still hate the Soviet Union are people who have been successful at uninstalling much of the brainworm malware, it's just that they haven't completed the process by uninstalling the anti-Soviet or anti-"tankie" worms... yet. And I say all this as someone who long considered themself an anarchist.
I think one does have to, to a degree, because you may have to conform with anarchists who believe this in order to work with them. Like how I have to keep my respect for Stalin close to my chest if I'm organizing with Trots. And if there's no one to organize with other than anarchists, Trots, and a spectrum of socdems, then... that's just what it's like here, lol (hyperbolic)