My opinion, Petty is not a dribble, drive, pull up shooter. He's more of what we'd call a set, jump shooter. Contrast that with someone like Steph Curry, MJ or even Larry Bird who could shoot a jumper but also shoot off the dribble.
The game when he went off early in the season and scored 30, IIRC, probably did him harm for the rest of the year. That game he was shooting 3s falling out of bounds and they were going in. I think he got lulled into taking what would be bad shots for him.
My guess is his mechanics got off slightly and much of that would probably include not having his feet set for what is comfortable to him and timing, which is a result of the speed of the game and trying to have a quicker release.
Edit: I haven't studied his shot much, but in the picture from the article, I don't like how his right elbow is not more under the ball. May be the camera angle, but it looks like that elbow would produce some sidespin more than backspin. In that pic, if we saw the follow thru I imagine it would not be directly at the rim, but off to the right of it.