Booing has long been a way for a fan to express his or her displeasure at something they're seeing from their team or one particular player.
Some of those people who choose to express their displeasure will just as freely and readily express their happiness as well. Some of the loudest booers are also among the loudest cheerers btw.
Of the cheerers, not all cheer at all the right things. For example they'll become the most involved only when the offense is on the field. Maybe they just like offense, or maybe they just don't understand the game. Same can be said for the booers. Those that booed Cassel then cheered raucously when O'C Their Savior came is an example of that.
Lack of knowledge about the game is a big reason for a lot-not all,but a lot- of misplaced reaction in either direction. But even knowledgable fans have different personalities than others-some are just more vocal-about everything.
Finally,some people have a mentality that supports 'punishing' and acknowledging the bad as much as the good.
On the whole I don't think Pats fans habitually boo because we've always seen the best from our players. I personally did not boo-never have and never will-but I didn't think what we saw on Sunday was our team's (or coaches') best effort. Some people let them know that, either by being non-involved in the level of play or being very involved in it.
Also, I think we're all getting out of our comfort zone a little bit with everything that's happened, not just Brady's injury but also last year's SB and the loss to the Colts the year before. Fans have been in a bit of an ongoing emotional turmoil; we all still feel the sting of that Superbowl but we pulled up our bootstraps and forged ahead anyway, looking forward to a new season which promised a changed-up defense and a re-charged offense. And with our schedule many of us thought we'd be back there for sure,maybe even undefeated. Then only 2 weeks ago our worst nightmare came true. Again, we buckled up and said "ok-we may not have our HOF QB but we've got the best coach in the league and a great team".
We beat KC and went on to pound the Jets, Mangini,and Favre.In THEIR house. We were UP. Again, "we may not have the best QB but we've got everything else we need."
Then it was Sunday. Now suddenly that great team and coaching staff we're counting on has not only made a few mistakes, it's just totally collapsed right in front of our eyes, from coaching on down. At that moment fans' hearts broke and a LOT of emotion was unleashed, all of it in different ways. It's been a long and difficult journey over these last couple of years for what's mostly a very emotionally invested group, and not everybody's going to deal with it in the same way. Our fans will be fine-though I do think if we have another poor showing out west, look out.
I'm sure I haven't answered your question but I did want to reply to because it was a good question imo, and it made me think. Thanks for posting it