Its not so much a "glitch" but an idiosyncracy. There's a special magic box "size" that you eventually learn in Starcraft. When all the units are "inside" of the Magic Box, they will move as a group, synchronously.
However, if even just one unit is "outide" of the Magic Box, they will all move individually.
The technique of "Mutalisk Stacking" involves burrowing a Zergling, and then making a group of 11 Mutalisks + 1 Zergling as a group. The burrowed Zergling is always outside of the box, and therefore the Mutalisks will "stack" on top of each other (moving as if they were individuals). You control them perfectly synchronized.
Because the Mutalisks are all flying units, they'll "stack" together.
Here's an example video of JaeDong's expert Mutalisk Stacking: https://www.youtube.com/watch?v=U4ipBj1sO3M . Notice how the Marines instantly die before the Medics can heal them... but the intense focus-fire and micro ability of JaeDong allows the Mutalisks to instantly KO Marines.
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In contrast, the Protoss player would use the Magic Box to perform Dragoon Dancing instead. In this case, the Protoss player wants to keep the Dragoons all together inside of the Magic Box (while the Zerg Example above was the opposite). You want your Dragoons to stay in formation during a Dragoon dance, which requires them to be "inside" the magic box.
Because the Dragoons are grounded units, they'll bump into each other as the AI pathfinding gets confused.
In contrast, look at how Reach uses these Dragoons. They are perfectly synchronized, because of the "opposite" effect of the Magic Box.
I am not sure if this exists in SC though
