I think it had to do with a girl, just not the way people immediately think of. I think Adam and a certain girl stopped getting along and a certain girl wanted more airtime. So she told Tim that if he didn't get rid of Adam, she would quit. Tim probably weighed his options and realized he'd soon be able to have guests sit in Adam's spot. He brought it up to Adam and asked him to leave, and Adam agreed because he doesn't like fighting with anyone and wanted to preserve his friendship with Tim.