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.