I've never understood how a guy could go from being so productive as rookie to basically being a bum the rest of his career. That always confused me.
The difference between the talent levels in the NFL is so minute, that even the slightest change in work ethic or preparation can mean all the difference.
Not to sound like Gregg Easterbrook, but the line between boom and bust is razor thin.
I don't know what exactly happened to Clayton, but it's not like he just stopped being talented. More than likely, opposing DBs adjusted to his tendencies after watching a full season of game tape and he was unable to match those adjustments.
To use a baseball example, it's like when Jeff Francoeur tore things up as a rookie and second year guy, then had his production fall off a cliff just when it seemed he was about to become a yearly MVP candidate. Enough scouting reports got around that he swung at too many bad pitches, and before you know it, he wasn't getting any good pitches.
Michael Clayton was productive in college because he had above average size and speed, but I wouldn't say either attribute was "elite." At the pro level, you have to have an elite skill to hang your hat on. Once DBs realized they could jam him at the line, he stopped getting open. Once he stopped getting open, he stopped getting his touches. With fewer touches, maybe he starts pressing, and then drops a few balls. From there, maybe the drops become a mental thing. After that, maybe he lost his drive to work out and lost a step, or maybe stopped watching as much film, or studying the playbook or whatever ...