Indeed it really does happen all the time. Human nature prevails in all things including sports. This is nothing new, it happens to every team in the NFL. Sometimes the better team wins of course, but no team is immune from having these types of games that can be reflected back upon as games where the "better" team didn't feel the need to get up for their opponent like they might against a more quality opponent. Maybe they think if they just go through the motions that should be enough to take care of the lesser team.
I don't think this is ever on any conscious level per se. It's just a collective feel the team acquires, perhaps even without realizing it the week leading up to the game. It's happened far too often over the decades to be an anomaly. It's dang near a weekly occurrence.
You have a 4-7 team coming into your house. You are 8-3. You've heard the narrative about the "NFL Least" for the better part of the season thus far. You have a perennial backup QB coming in that has never scared a single Soul in the NFL...It's human nature to assume just by showing up and going through the motions a victory is almost certainly assured.
Then the game starts. The other team is playing for their very playoff lives. You are shocked, trying to steady yourselves, match their intensity and before you can even get the license plate of the Underestimated Truck that just ran over you it's too late.
Hell yes they underestimated the Giants. It was obvious virtually from the start. And the sad news? It's gonna happen again. And again. And again. It's not a Seahawks thing, it's a human thing and it happens All. Of. The. Time.