I disagree that it's just the octagon factor, in Pride he had drive, fire in his eyes... his UFC matches he looked totally out of place, like he didn't want to be there like it was a chore. I just think he's lost the drive and motivation to try anymore, that plus he underestimated his opponents, he thought he was just going to waltz in there and go right for the title.
Basically he got tired/lazy.