It's not really a shield storm as much as the effect was intended to show
the electrical discharge or "frying" of components as the energy raced along the
hull... not the shield.
You're referring to the effect just after this scene....http://www.emergentchaos.com/images/05-nov/ion-cannon.jpg
Of which I'm fairly sure you can see the same effect duplicated in the Gungan
land battle at Naboo in EP 1. The blue orbs caused the little electrical effect as well.
So really - it's not the shield, it's a hit effect on the actual structure being attacked.
The ion cannon on Hoth was supposedly similar to a ISD or comparable ship to ship ion cannon
from what I've read. Just with a much greater distance. If memory serves - that ESB scene -
the ISD took 3 or 4 hits to be disabled and the ion bolts had no visible effect when hitting/passing through the
However, besides the ion cannon vs. hull "ion storm" there was a part in one of the Xwing books
where Wraith Squadron had made a hole in a ISD's bottom generator dome - and there was an energy
discharge or "lightning" effect mentioned going on amongst the components within the hole.
I think the "shield hit by laser/ shield hit by ion cannon" temporary electrical storm was probably something else seen in the game
cutscenes as well.... simply again for effect.