maybe they always have been and I've just not noticed it before
As someone that was a 'nerd' back in the day before Big Bang Theory made 'nerds' cool, I'm gonna say that they always have been and you just haven't noticed before.
The nerd / tech crowd have always been a bunch of miserable little sexists and dorks with a superiority complex. And half the time that little Timmy got stuffed into a locker or dunked into a toilet, he fucking deserved it. And to me, it's really no surprise that they're homophobic and transphobic too.
If Big Bang Theory was actually accurate, Howard and Raj would be prolific posters on incel forums that whine and gatekeep every time the girls come over and then later whine about being lonely, and Sheldon would be borderline nazi. Not sure about Leonard because even without changing anything about the show, he's still the only half decent one of the lot.