He overcomplicated the offense. From all accounts, the Thanksgiving bye "heart to heart" was all about simplifying the route tree and concepts.
I wonder how Winston would have done if he'd had the same pull as Brady. I'm not saying he would have been an all-pro quarterback, but I don't think throwing 30+ interceptions is entirely his fault either.
There's an SI article from a few years ago that details a week of Carson Palmer (then with Arizona under Arians) preparing for a gameplan. It's completely over-the-top and an absurd amount of variations Palmer has to study for; in addition, there have been other citations about quarterbacks (Andrew Luck, Ben Roethlisberger) struggling badly to comprehend an Arians offense, especially in year one. It was obvious that Brady was trying to dig too deep when it wasn't even necessary, having such great weapons around him and a capable offensive line.
Hopefully Arians learned a lesson...you can have the greatest gameplans in the world, but if the players can't all be on the same page in executing them, and if their complexity causes the offense to slow itself down, they're not that great after all.