I'm sure blizzard is well aware of the ground slam bug, and just doesn't know what is causing it.
It appears as if we do not know either, but the leading theory is something to do with targeting and line of sight. It should be fixed though obviously. Gruul without shatter is like killing vaelastrasz without burning adrenaline.

Theory
Gruul targets 1 or more spots with an AoE knockback (wingbuffet type effects).
If one or more of these spots are out of his line of sight, he fails to cast groundslam (and the resulting shatter).
If YOU are out of line of sight of one of these effects, you dont move (leading to the strange behaviour where people dont move when near random objects in the middle of the room). Here I refer to people who ae not moved at all, and NOT people who are moved but bounce off walls or other bits of scenery.

NONE of us were flasked. We havent flasked since our very first kill. I have to pace and soulshatter twice as-is. Flask would be meaningless for me.