Easily. The pressure of his new contract, and the sudden, newfound position he found himself in. He went from being a funny stand up and part-time movie actor to being called a basically brilliant entertainer whose skits did a masterful job of portraying some societal issues that most people didn't have the balls or talent to tackle and make humorous. I think it was a case of "holy shit...there is a lot of stress on me to put out a fantastic season of shows...what if I can't do it....what if they say I wasn't worth the money...what if nobody loves me anymore....what if I stop being funny...etc....."
Basically a downward spiral of self-doubt and pressure.
But that is just my own theory...drugs could easily be involved.