Could be a sign they think the Tories won't win next time. Might be people standing down who think they have no opportunity for higher status jobs under Sunak. Could even just be that they want to return to their original more lucrative professions, or are hoping to cash in on their government experience with very well paid jobs in the private sector before they are too old.
I'm not reading anything significant into 13 Tories standing down, it's probably a mixture of several things. I wouldn't want to be away from my family for half the week for years on end either.
Crispin Blunt's going though so that's good.