My understanding is that was because Heard couldn't read a defense and Gray was the one doing it and calling the alignments.
At the elite11 camps Heard won the "diagnosing defense drills" with both his speed and accuracy of picking the D out. Just saying. I didn't hear anything about his job doing so at Texas, but I agree with some of the other guys above.
Namely the OC changes and style are more to blame. We rolled out stripped down versions of an O, with a makeshift OL, and a young QB who preferred like most to scramble when pressured. Sure Heard has plenty of room to grow in running an O, but its not over for him just yet.
People overreacted to his great game vs Cal. And people are overreacting to him predictably struggling as the year went on. We will see if he overcomes it and shines or if someone surpasses him.