Same thing as everything else on the internet, it started out as a joke and snowballed into what it is today.
Not so much in this case, 15 years ago black jokes started off as simply shock humor on places like 4chan, anti-Semitism on the other hand was as fringe as fringe can get in the mid 00s, the anti-Semitism I saw back then, which was a lot rarer, in my opinion was fueled by more than just a desire for shock humor and you simply saw more black jokes.
You have to remember 4chan 15 years ago was a very different place, most users were generally pretty anti-Bush and generally pretty favorable of Obama early on, yes people would make offensive jokes for shock value and yes you had some legit racists back then, but it wasn't like today.
The anti-Semitism that has cropped up in recent years has been fueled by people simply noticing things, especially as Twitter took off and Jews started to play the "dear fellow white people" game, unless you're a spineless loser that literally hates yourself and hates your own people, you can't help but notice a simple fact, Jews have played a big hand (how big is up for debate I guess) in pushing toxic rhetoric that is at it's core racist against whites and has done nothing but broken down America's social cohesion and made the country a worse place and innocent people have suffered, no one has the right to hurt innocent others no matter who you are or what you're excuse is.
Now with that said I don't necessary buy into the far right narrative entirely, I think a lot of what's been going on is simply bad side effects of the way social media functions and encourages identitarianism, tribalism, paranoia, boiling complicated things down to simplistic talking points and other toxic shit, but if you're Jewish and you're spouting toxic bullshit, you deserve to be called out on it, no special exceptions.
The only way a diverse, multicultural and multiethnic society is going to work is if everyone is playing by the same fair set of rules, not one group of people are beyond reproach and criticism and another group of people have no voice and can only be criticized constantly no matter how unfair, it's in Jews own interest to stop drinking the divisive Woke Kool-Aid otherwise if they keep pushing rhetoric that seeks to harm my people, then we got a problem big nose.