This was a change in preperation and production, he basically jus captured all the footage live to get the series over and done with. It seemed to happen a lot like that with his longer projects, I think it was FF8 that always disappointed me cos he did like 2 episodes for the last 2 discs. 13 may have been similar also.
I wasn't a megafan, but I don't think Spoony had declined or changed noticeably in 2013. For me it was 2014/15 where it became obvious the show was pretty much over. With the Massacre at Central High and Reb Brown Love Boat episodes being the memorable moments I realised he wasn't the same and it was dying.
If you want an answer to what caused the dramatic change, for me it was simply Patreon. Everyone adds other things into the equation, but I remember seeing him announce his Patreon and I knew right then it was game over. Other factors didn't help but as soon as he was getting paid regardless of his already sparse output, he was done for.
It's a shame he let a great body of work go out in a limper but I'll always enjoy his reviews 07-13