This may be hard to believe, but psychiatrists and other prescribers actually do use the term "antidepressant poop-out" among themselves. Not very professional sounding, but it has high communicative value! This phenomenon occurs fairly regularly (thanks farmergirl for the 8% figure, I have previously heard about 20% for poop-out), and they just don't know what causes it. As JD wrote, it is not caused by tolerance. The mechanism is unknown. Switching ADs is the typical solution.

I have heard this too, that ADs acting on more than one neurotransmitter have less chance of pooping out. So SNRIs and TCAs poop out less frequently than SSRIs.