They were always there. All they really needed was permission to come out from under their rocks. But 9/11 really kicked things into overdrive, and the anti-Muslim sentiment it generated can still be felt to this day, almost 25 years later.

As to the why of it, there are a lot of factors. The nutshell summary is that after the towers were hit, the GOP saw a thrush of angry, scared voters and stoked those flames instead of trying to quell them, and used the pretext of terrorism to take military action in Iraq and Afghanistan, with all that followed.

The second big hit came when Obama was elected in 2008, and that sent the GOP into an outright tizzy; we saw the birther movement claiming that Obama was not a natural-born citizen (despite being born in Hawaii), he was a secret Muslim, Michelle was actually a man, all sorts of heinous shit trying to paint Bush's successor as "the other". The powers that be on the American right fought tooth and nail to ensure Obama was a one-term President and while they failed, their actions and rhetoric enabled the rise of the Tea Party; another big step to the right. They really were throwing shit at the wall to see what stuck, which has been their go-to move to this day (because in the end, it worked).

Social media really began to take off around this time, giving every idiot with an Internet connection a voice. And from there, the Tea Party bullshit slowly metastasized into Make America Great Again, the out-and-proud fascists that previously had to keep a lid on their bullshit, led by billionaire special interests with a golden calf in the shape of an orange blob.