Imho two things happened :
#1 - Denver's schedule had them playing a good number of weaker teams (even my mediocre Cowboys!), which led to a somewhat inflated sense of how good Denver actually was.
and ..
#2 - EVERYBODY (well, it seems like everybody, particularly in the sports media) would go on and on and on and on and on about how Denver was the best ever, Manning was flawless, they were unstoppable, etc. It seems like they might have started to believe their own press, which is a dangerous thing. It's like they came onto the field only feeling like they had to physically show up to the game in order to get those rings and that trophy, when in fact they should have showed up ready to play a tough game.
If they were mentally set for it, I think they could have made it a game. And for that, I put the highest level of blame at the coaching staff of Denver. The message should have been :
"Forget everything anyone has said about us. We are 0-0 headed into this game. We are getting ready to play a team that has a wild hunger and tons of talent who will play at 110% every single play. We need to play at 115%. Those tricks we use to confuse the defenses aren't going to be enough here. We simply have to be faster, stronger, and have more heart to make our stand, and to claw for those yards."
Instead, we saw a game plan shatter completely in the first few minutes, followed by a systemic failure to make any adjustments at all. Nobody emerged as a leader, nobody adjusted, and even more mistakes were made flailing around on the field like a bunch of flabby drunks posing as NFL players. It was the most complete beating I've ever seen in the postseason.
Nobody can take away the Seahawk's supreme performance, but I still feel like a truly jacked up ready to play Broncos team could have made that a 34-23 kind of game instead of a complete annihilation. Even the sole Bronco's touchdown looked like charity, haha.