There is a technical discussion at this thread in the China Shop that explores why pH rises and what can be done about it. It's a long thread, but the bottom line is that the pH normally rises due to the outgassing of carbon dioxide. Pools are intentionally over-carbonated (like a tasty beverage!) as part of the pH buffering system from carbonates. When carbon dioxide outgasses (i.e. leaves the pool water and goes into the air) it raises the pH of the pool water.

This outgassing process is accelerated by aeration of the water, by lower pH, and by higher TA. This table gives a rough idea of relative rates, but aeration isn't part of the table so the actual rate of pH rise will be depend on the aeration. An SWG actually aerates the pool as the hydrogen gas bubbles that are produced are a form of aeration and pull out the carbon dioxide faster so typically SWG pools have more of a problem of rising pH than non-SWG pools.

The two easiest ways to reduce the problem of rising pH, other than reducing the sources of aeration, are to lower the TA using Ben's Lowering Your Alkalinity procedure and using 50 ppm Borates (from Borax) if you have an SWG since that lets you lower SWG output since the Borates are an algaecide so do some of the work that chlorine would normally do. You can also not try and fight the rise as much and keep your pool at a somewhat higher pH.

Richard