pH rise in SWG's mostly comes from the outgassing of carbon dioxide caused by the vigorous bubble formation of hydrogen gas at cathode. The gas bubbles are pure hydrogen gas and the CO2 wants to diffuse into the bubble (higher concentration of dissolved CO2 in the water than in the gas bubble). So the bubbles generated by the SWG carry away hydrogen gas and CO2. Because of the net loss of CO2, the alkalinity of the water shifts in equilibrium to counteract the effect and, in the process, consumes a proton (H+). This is what causes pH rise in all pools (aside form adding that chemicals that cause pH rise like washing soda).
However, the SWG effect is somewhat small. Typically what people are seeing is not some much the use of the SWG causing the pH rise BUT, the discontinued use of trichlor tablets and dichlor powder which are acidic. The acidity of the stabilized chlorine compounds both lowers the pH of water directly and consumes alkalinity. So, people who are used to using stabilized chlorine also get used to the fact that the pH tend to be more balanced due to the fact that the acidic solid chlorine is offsetting the natural aeration of CO2 from the water. In some instances, these pools can actually have no net pH change over an entire season.
But you have to ask the question - what would you rather do? Add acid to your pool on a more frequent basis or drain your pool when it becomes over stabilized by too much CYA? Adding acid is far, far easier because fighting pH never causes an algae bloom to occur. On the other hand, fighting with high CYA can easily cause an algae bloom and thus compound the problems and costs associated with using solid chlorine.
