Here's an article, published a few weeks before Dourdan's arrest, that says he "decided to quit the show at the end of the current season".

Dourdan leaving CSI - TV Squad

I guess he made enough money from it so that he can afford to nod out wherever, and whenever, he wants to. Or possibly the studio knew of his drug problems and gave him the boot, but let it look like he was retiring on his own.
Breukelen advocaat Reviewed by Breukelen advocaat on . 'CSI' star: Felony drug possession Don't know whether this should go in the TV/Movies section, or here. I guess he can kiss his gig on CSI goodbye. 'CSI' star Gary Dourdan charged with felony drug possession 1 hour ago PALM SPRINGS, Calif. (AP) â?? "CSI" co-star Gary Dourdan has been charged with felony possession of heroin, cocaine and ecstasy stemming from his arrest last month, prosecutors said. The 41-year-old actor was arrested after police found him asleep in his car in Palm Springs on April 28. He was Rating: 5