We may have Maxine and the rest of the gang to thank for this. I don’t think he was planning to go but these people have shown us who they are and he is doing his part to protect the country.
Alternately, he might think that his replacement now might be fairly moderate, in order to get through the divided Senate. In 2019, though, we would either have a conservative majority Senate, where Trump could push through a very conservative choice, or a dem controlled one which won’t approve anybody from Trump.