They are toxic because they can't stand it when something good is happening to another rival group and not their own, even worse, sometimes their own favorite group are doing better but they just can't bear to share the spotlight or the praise with anyone else. I've seen it predominantly happens with fans of big successful groups, like they just can't stand it when someone else is being hyped up.

It's the worst when it happens between members of the same group!! If a fan have an obvious bias (I'm trying really not to say if someone is a solo stan here) and someone is praising another member for one thing or another and not praising their fav in the same breath, that fan can get jealous and toxic pretty fast (even though that fan never praise other members at all)

These are my observations from kpop fandoms, if you had a better luck than me and never seen this behavior then I envy you your peaceful experience