Young created a ton of chances for the bigs who set the high ball screen and for the shooters on the wings. Early in the year, these chances were converted at a nice rate (in particular at the 3 point line when the team was shooting a tick over 40% from the 3 point range), but later in the season the conversion rate waned. This frustrated Young, which definitely showed. Also, defenses changed how they attacked Young and this action. Having said that, one thing that Young performed poorly on in the season, especially the second half was finishing at the rim. His FG% at the rim was way too low and that affected the team significantly. It also didn't help that when he did pass to the bigs/cutting wings in the lane off dribble penetration, they failed to catch the ball or missed way too many bunnies.
The defense was mediocre throughout the season, but ultimately this season flopped because the team quit converting offensively.
As for ball movement, I do agree that OU didn't do a great job on secondary action, but where it really waned was in tertiary action. Early in the year they really never went that far in the offense because they didn't need to because they were converting so early in the shot clock. There is a reason OU was averaging a high points per game. Their pace and efficiency were high. Unfortunately, the efficiency went down as the season progressed.