Because I get tunnel vision when I'm tanking this encounter, I really have a hard time checking this myself. I was wondering if the cave-ins that Gruul dumps on you eat shield block charges. I tend to at least get one or two crushes, sometimes at the dicey high-growth portion of the fight. I'm pretty fastidious about keeping up Thunderclap and Shield Block, though my system is a bit on the slow side and I wonder if bottlenecking isn't causing a bit of client-server lag between the time I hit Shield Block and the time the server recognizes it.
During the early parts of the fight I'll just eat the cave-ins and I've never noticed it eat Shield Block charges.
Shield Block lasts 6 seconds, but it has 5 second cooldown. When it has about 1~2 seconds left on cooldown, just mash that button to ensure it stays up. You can even delay some of your other abilities to ensure you'll have enough rage.
Make sure to keep an eye on Melee and those Pets. I had to literally yell in vent telling them to get behind Gruul. Sometimes people just mindlessly push buttons and forget about the consequences of not paying attention.
During Shatters I almost always run away from Gruul, mainly because our Melee just stays there DPSing and I'm not willing to kill them and myself. But I did read that if you're right next to him during a Shatter, it's very easy to get crushed after it breaks since you probably won't have enough time to hit Shield Block before he hits you.
I should clarify that I usually land in the middle area, about at the range healers stand, and only intercept to him if I'm expecting Hurtfuls rather than being his primary target. What I meant was that it's possible that either a melee parry happened during the time I back around in a circle to get him into the middle again, or I was slightly out of sync while moving him and he got "behind me."
It's very possible that you turned your back in such a way, that the angle produced between yourself and Gruul was your back. Which is why I never try to run through him or strafe trying to position him, I try to move around his radius and then run backwards while facing him.