There's good toxicity and bad toxicity.
Good toxicity is gamesmanship. You talk poop in the arena -- including forums -- but it doesn't go past there. It's understood it's not real. The same should be true in sports. If it makes you really mad, that's on you and you should take a break. In every game I play, I love having rivals. I enjoy having a faux dislike for some opponents.
Bad toxicity is what I see in a lot of screenshots these days. Raging, baseless accusations, spam, harassment overt bigotry, racism, misogyny, etc.. This kind of toxicity can escalate into RL via threats of harm, including but not limited to physical harm, doxing, swatting, etc... Unfortunately, there are a lot of troubled people out there.
Who remembers the extensively long stories that @Publius_Scipio used to write about different players? Those were awesome..
I think it often starts with the OP's choice of subject or headline. If you start off trashing someone like all the "Fake Tank/Healer/DPS" etc. or the Nerf Crowd that always has a problem with something or how you for instance are talking about the "good ol' days" it gives a sense of EFF-Yew to new people or those being called out.
People are looking for confrontation.
Trolls are like Beatle Juice, If you call their name, they will answer you.