I'm pretty sure it's just physics. If you have an object moving forward and apply a smaller force to the right, the resultant force will be diagonally to the right... your ship won't travel to the right very far.

After a quake/missile/rocket hit you're ship has very little forward thrust, therefore the sudden SS boost you gain will have a bigger effect going sideways. That's why you shift the entire length of the track when you're near stationary.