I've seen it happen a couple times. They get everybody else to help them, so their assignments get done. They invite the CIO out to lunch every couple weeks so they're insulated there (helps if they already know the CIO).

Out of desperation, their immediate supervisor assigns them some non-programming task like kicking off a special report every morning. The manager resigns and now they're the only one with institutional knowledge about how to do that task. So the new manager isn't going to fire them. Besides, the CIO speaks highly of them, so they can't be all that bad…

tl;dr: politics