When and why did Matt Harvey go off the rails?

I've wondered about that for years, and have had my theories, but I was never sure

in my opinion, a May 2017 piece by Tom Verducci, provides the smoking gun.

As a high school senior, Harvey threw 91-95 mph and his 6'4", 195-pound frame had him described as “lanky.” Two years later, as a sophomore at UNC, he weighed 255 pounds, showed inconsistent velocity and posted a 5.40 ERA.

(He later admitted he gained “a lot of muscle” that season through “some stupid workouts,” which presaged the yo-yoing of his weight in the big leagues.) He rallied with a big junior year to become the seventh overall pick of the 2010 draft.

Along the way, building upon the mechanics he learned from his father, Ed, who taught him to “show the ball to second base” on his arm swing, Harvey tweaked his mechanics to generate more velocity. “I’ve got the higher arm slot,” he told me, “so for me to get up and let my arm work I have to really stay back a lot longer and get my arm out a little sooner. At this point it’s natural. When I was learning, I always pointed to second.”

Pointing the ball toward second base was commonly taught, though some pitching experts today believe it creates more stress on the shoulder and arm, preferring a righthanded pitcher, for instance, to “show” the ball more toward third base or shortstop in the load phase.

Moreover, to get to his high arm slot, Harvey loads the ball far away from his head and so high that his right elbow gets higher than his shoulder, creating a stressful position known as elevated distal humerus. But if you can throw a baseball 100 mph, where is the incentive to change? The incentive only comes with failure or injury, which have brought Harvey to this next phase of his career: the transition phase.

Verducci again mentioned this in his 2018 piece Matt Harvey's Quick and Agonizing Decline With the Mets Is No Mystery

The decline in his stuff was obvious. And there was no way his fastball was coming back with the way he throws.

Harvey pulls the ball far behind him—crossing the airspace over the rubber, a strenuous maneuver that rarely leads to long careers.

“My dad taught me,” Harvey once told me about his signature arm swing, referring to Ed Harvey, a well-regarded high school baseball coach in Connecticut. “He said bring it down, show it to second base and then go.” The pushback arm swing, the “showing” of the ball to second base, and the way he raised the ball high and far from his head helped Harvey gain velocity, but over time those maneuvers also strained his arm.