Why does water surge out of the shower head after being turned off?
By surge of water, I'm guessing you mean a quick trickle that would fill a cup and not a hard spray that would let you wash your hair. Like Steven says, the shower head will still have water in it after you shut off the water. 

My guess for why you see the surge is that the water is held in the shower head by water tension at each of the nozzles. Eventually that breaks from vibration, water contracting as it cools, or drying out. This allows air to be sucked into the shower head while water drains out the rest of the nozzles.

There's nothing much to fix. But if it bothers you, give it a shake or run your finger over the nozzles to break the surface tension. Another suggestion is to make sure your shower head is mounted at an angle, rather than spraying downwards.