I think were booing Carr for some decisions he was making, but I also think the fans were getting frustrated.
The boos were unmistakeably and vehemently directed at Carr (the player) as he walked back on the field to try and salvage what he could from the disaster. The boos weren't directed at a failed play or a play call. They were directed at him. He's part of a bigger issue with the offense but he will naturally catch all the flak because he is the QB and a signature player of the franchise. People can boo all they want, but "great" fans don't kick their own when they're down in my opinion. But at least it shows they care, I guess.