He obviously disagrees with the direction Trump is taking the legal strategy in, with the hiring of Di Geonova and his recent unhinged tweeting about Mueller.
It shows that Dowd thinks it's a bad idea to take on Mueller. With a bit of inductive reasoning, you could be forgiven for thinking that the only reason we're at this point is desperation on Trump's part.
If Dowd thought getting rid of Mueller was in any way a viable play, he'd still be on board. I don't believe for a second that he didn't reach out to Trump allies to gauge the mood in the House and Senate over pushing Mueller out and judging by the reaction it didn't go well.
With the investigation reaching the areas it has and results it has already had, I think Trump's options are very very limited now and going down swinging, with the hope of dirtying the reputation of Mueller and his investigation's is the only play left.
I still think Mueller has some cards up his sleeve. If his investigation is canned, I expect some serious indictments to be unsealed by Schneiderman and co that take down either Trump or family members and force the investigation to be reopened.