Looking at his Gruul performance, this tells the whole story I think:
https://www.warcraftlogs.com/reports/2FkbwTM9AP4Y3QZr#fight=1&type=casts&source=13 (select all the casts)
There many periods of inactivity. One is over 10 seconds. He needs to always be casting something, even if he's moving away after a petrifying smash (the biggest holes are due to this). The rule: instants on the move (even if it's just refreshing flame shock early) and hard casts as soon as you can stop moving.
This may be due to him losing track of what he should cast next and stop to think. This is