Normal swimming pool TA levels cause the water to become over-saturated with CO2, essentially like carbonated soda. When CO2 out gasses the PH goes up, CO2 is carbonic acid in the water, when it goes away that causes an increase in PH. In a standard pool with no SWG or water features, CO2 outgassing happens quite slowly and can be ignored. But anything that aerates the water creates opportunities for CO2 outgassing. SWGs produce small hydrogen bubbles that are effectively aeration, and encourage CO2 outgassing.
The rate at which CO2 out gasses is a function primarily of the PH and TA levels, along with the total amount of aeration and some other smaller factors. High TA, low PH, and more aeration causes more CO2 outgassing, and thus faster PH increases. In a typical trichlor tablet pool this whole effect is drowned out by the acidic trichlor tablets constantly lowering the PH. But in a liquid chlorine or SWG pool there aren't any constant acid additions to drown out the PH increases.
With TA around 60 and PH around 7.8 you can usually slow the rate of CO2 outgassing down enough so it isn't significant (though additional aeration can make even those levels insufficient). At higher TA or lower PH you tend to see the PH drifting up. When the TA is fairly high, say 150+, the rate of PH increase can be rather dramatic and very annoying. In more extreme cases, the addition of borates to the water along with lowering TA down to around 50 will usually be enough to stop the PH drift.