Basically, Rick started using a lot of coke.

And he also got lazy - he hadn't really contributed greatly to the music since WYWH.

During the sessions for The Wall, Wright was given lots and lots of time to come up with some keyboard or piano parts, and he contributed nothing. He was kept on for The Wall tour, and was credited as part of the band, although he just filled out the role of live player. Rick claims it's just more evidence of Waters' megalomania though, so it depends on who you believe.

__________________
I'd always thought Clive was a bit odd, but when I found him hanging from a noose from the ceiling, I stopped talking to him....

I'm Satan, and you're a fool because you're buggering up your turntable.