So, they got supremely confident and started believing their own hype.
Then they watched their next opponent trade away their best player and give the starting QB job to the third string QB, who had only started one game in his entire career before that week.
Sure, the Browns won their game that week, but it was an ugly win against another winless team.
Simply put, they got cocky and treated the Browns game like a guaranteed win. They probably just went through the motions at practice and didn't think the Browns were a legitimate threat to completely shut them down like they did.
Obviously, they were wrong.
What needs to happen now? Well, for starters this embarrassing performance should illustrate to them why you take EVERY game seriously in the NFL. This isn't the NCAA where you'll play games against teams you outmatch in every category. This is the NFL, where even the worst players are better than 99% of the college players out there.
If you get cocky and act like there is no way you can lose a given game, you are almost guaranteed to lose that game. And I think our Bengals just got reminded of that in no uncertain terms.