I've been on Fisher's side of this for a long time, and have believed in him each year.
At this point, what you're seeing is that, including many of us who have been staunch in our support for the guy, a large portion of the fanbase is sick of these kinds of seasons. Seasons where we seem to have all the pieces, and the coaching staff certainly seems to believe we do as well, and yet we underperform, our plays misfire or are poorly executed, we get buried in yellow flags... And by the fifth year, enough is enough.
I don't think it needs a lot of explanation. We're tired of being a laughing-stock, and we feel this coach has had enough time to fix his philosophy, adjust his approach, and has shown an almost arrogant refusal to do so. So we're mad. We're mad at seeing the same problems week after week, and hearing excuses from our coach, not ideas. Our coach is hoping things work out. He doesn't know what the problem is, he's just hoping the players play better.
So we're sick of it, we're mad, we are scared that if we don't voice our displeasure, no one will get the message, and we'll be saddled with another three years of this. We all want this team to succeed, and we would all be over the moon if Fisher got them there, made the play-offs, made a deep run, and did so for multiple years, and hopefully got a shot at a superbowl. It just really looks doubtful, and there doesn't seem to be any reason to believe more Fisherball is going to get us anywhere. So. I don't know. I think the negativity right now is a product of caring about the team, and not being satisfied to tread water, and see the same issues sink us over and over and over.