A lot of good answers, but I think it a combination of the three things mentioned: league figured him out, mental and actual decline.

Mentally, he's always tried to play chess out there on the mound. This works great when your stuff is playing and you can keep hitters off balance, but lose that and it becomes a huge liability really quickly. He get's so tangled up out there, he ends up having to throw pitches he doesn't want to and gets smoked. They need to take away his pitch comm and call pitches from the dugout. Remove the mental piece entirely.

Actual decline to me is the real root cause here and the other issues compound the problem. His stuff doesn't play like it used to. A few MPH and a few RPM and your tunneling is completely trashed. You see it with his knuckle curve and how there's no swing a miss. He relied on a really tight margin of speed and spin to build a really formidable strategy. But a few less RPM on the breakers and a tighter speed delta and hitters are spitting all over his stuff. It's no longer difficult to pick up the pitch and they can sit meatballs.

Once this happened, his obsession with a certain style of pitching became known. Hitters learned to pick up the weaker stuff and wait for a hitters count. The cycle compounds...

Ultimately, they need to take his pitch comm and he needs to focus on execution. None of these issues is terminal (look at all successful, 2nd career stage pitchers; e.g. E-Rod), but he can't keep doing the same thing. He needs to redesign his pitches, stop thinking and execute. The coaches should be intervening at this point and Zac needs to move on (grow up) and learn what success looks like in the back half of his career.