I think the drop in quality recently has more to do with his spending less time polishing each individual drawing as well as stylistically shifting.
Over the past few years, he's changed styles several times, usually pivoting from tightly controlled linework (muscles, expressions, etc) to looser, more casual linework and back again. Personally, I prefer the tighter linework since it keeps the anatomy in check (within reason) and generally looks better. Of course, polishing like that takes time and the style he's using right now uses much softer, rounder, flabbier, even incomplete lines. Plus, there are many more sketches now (and all sketches are usually loose) with less time to develop them, along with fewer pieces getting fully finished, which is all he'd post back on y!gallery.
All this to say, I don't think it's that his ability has decreased over time (that Free! orgy from a while back has much more controlled linework and looks great), just that he's currently using a different style and, from what I blindly assume, is spending less time polishing each piece before posting them. That makes for quantity first. I also don't think he's taken anywhere near as many commissions as he used to, meaning he's had more flexibility to loosen his style to what it is now. In the end, not all styles appeal to every person's taste, though I look forward to tighter linework again.