Ok, here is the REALLY simple answer. Your pool is carbonated, we call the measurable part of this (which are carbonates and bicarbonates) TA. The non measurable part (by our testkit) is carbonic acid or carbon dioxide dissolved in water (not completely the same but for our purposes here they are). You might know carbonic acid by it's common name, club soda or seltzer water. When you shake up a bottle of club soda (areate it) it goes flat (carbon dioxide outgasses). You have now decreased the amount of carbonic acid (keyword here is acid) so the pH rises.