What makes a unit unburrow with storm? does it need a certain hp? Or is it decided by wether the unit is hero or not? or maybe how much of its hp is left out of the total?

i've tried looking through so many different maps and i can never seem to pin point why a unit unburrows/doesn't unburrow when stormed.

Heroes have no inbuilt AI AFAIK, therefore they don't react to being attacked when burrowed.

well from what ive seen they still unburrow for storm.

Maybe for spells, but for normal attacks I had a invincible Hyperion attack a hunter killer and he unburrowed fine.

Hm, heroes are ignored by computer AIs, but that's unrelated to the actions that units will automatically perform when attacked, like trying to escape.
I might be wrong but I think that they should unburrow.

It merely needs [I think] be inside the vision for it to always unburrow. If your templar is outside the burrowed unit's vision, I think it tends to stay burrowed, unless the vision is obscured by a cliff or such. Also, if the templar casting the storm is friendly, it will never unburrow.