I could be wrong but it seems to me the avalanche of negativity started when teams became able to interact with him/his camp.
His play can't rationally be faulted to a high degree so "the private interview" became the only excuse. Even Mahomes said it was "Crazy" Shedeur was dropping. Shedeur was among top 10 QBs 2 years in a row. He had Travis Hunter and another decent WR but that was it. No decent RB or OL.
He's also a pocket passer and they are still be best QBs in NFL. Lamar Jackson is a spectator like the rest of us for Super Bowls. So Shedeur will hold a ball vs being a panic chucker tossing INTs. He took brutal hits (many cheap) and always got up and hung-in again.
Shedeur is actually a very mellow fellow. All the "brash" stuff is excuses. The Sanders want to win and they play/coach hard. Most NFL coaches (and fans) are lost at QBs/offense these days, and they like to keep their comfort zones with the same weekly excuses (like Schiano, Dabol et al). Many of the best QBs had fathers who knew how to coach their kids young (like Mahomes, Elway, Luck). In many ways Shedeur is better for the NFL game than the CFB game.
Of course the NFL QB game has been going to heck (as Brady pointed out) and many NFL coaches probably want plug-n-play robots and Shedeur is smarter than that sort of coach
"I think it's been dumbed down a lot," Brady said. "I think it's just, they're taking the quarterback position where you should've been the field general, you should've had a real say on the field and contributed -- you know, you always had the last swipe at the pencil. And I feel like the coaches now are trying to make it more coaching-focused and just [saying] 'Run what I call and get out there and be a robot, and then adlib if the play breaks down.'"