Fandoms often contain people who are genuinely fanatical and the structure of social media platforms like TikTok fosters a lack of reasoned debate with people of different opinions. So you get the so-called "echo chamber" effect. And mental illness causes particularly strong feelings for a variety of reasons, many of them very valid. So, if you combine those things you have a recipe for toxic elements to magnify their worst traits exponentially. Especially when it comes to people with a dissenting view.

I've struggled with moderate to severe mental illness for over 20 years and more than once I've had to catch myself from falling into that automatically defensive stance whenever someone disagrees. And I didn't get into social media until my mid-20s. Like you said, people who are constantly online from a young age can easily find aggressive defensiveness being their default/only coping mechanism. Even people suggesting a more moderate response to disagreements can get torn to pieces for supposedly implying they're "making light" of the severity of people's lived experiences. Social media is amazing, but we're still very much in our infancy as an "online species" and it really fucking shows whenever you toss a little dissenting chum in the waters!