This one feels a little different. He stated previously that he was going to stand down at the next election, which is reiterated in the article:

He said he would not seek re-election to the House of Commons at the next general election. But, writing in the Observer, he says he envisages a role advising the Labour party on its policies on mental health while focusing more on his NHS work.

Defecting - and especially co-ordinating with Labour for months to time his defection and pre-arrange him joining Labour - isn’t just fleeing the sinking ship so much as hanging around a little longer and deliberately trying to make it sink faster. It represents something rather than just being about saving his own skin.