Some just get sick of the same drama over and over. Their passion dries up, as the opportunities in the space do (not campaign related). I found less opportunity to talk about what brought me here, or people to assist in ways I was capable of doing so.
Then there is the fact you build real relationships with digital presences and those just dissapear sometimes. The slightest pull from the outside world can then make logging in one day just seem not worth it.