Kinda surprising because it's obvious Druckmann's real goal is to be a pretentious movie/TV director a la Kojima.
Or maybe it's bullshit and in reality the show is falling apart because viewership collapsed and he's just jumping ship.
I have no idea what the interpersonal politics are, but I'd wager that there was some type of disagreement between Druckmann and Mazin born from the fallout of the collapsing critical reception and viewership.
Imo the most engaging (and interesting) part of TLOU2 are Ellie's chapters in Act 1, there was no reason for the season to shit the bed so hard, and it seems like Mazin's ignorance (or misinterpretation) of the source material had a big part to play. The episodes Druckmann wrote this season were the highlights (relatively speaking), and he obviously tried to write Potato as close as he could to Ellie despite Mazin undermining her character. If season 4 ever happens, I'm really curious to see how they'll handle Act 3. The entire Santa Barbara section didn't make any sense within the story of the game, I can't imagine how they'll handle it within the framework they've established in the show.
Mazin and Druckmann are egocentrists with delusions of auteurship, with very different creative visions and goals, fighting for control over a property that is quickly becoming worthless. Mazin obviously won the bout. I get the feeling Druckmann's getting out while he can before his reputation (such as it is) becomes even more tarnished. Give it a couple years, he'll be back with some other "prestige" television slop, but as the sole creative behind it this time.
Also, I just looked at Mazin's writing credits. People seem to regard him as some visionary genius because of his work on Chernobyl (which I haven't seen), but that really seems like a fluke because most everything else he's done is objectively dogshit.