The more I read your post and think about what's happening, I'm pretty sure im in the ballpark of what's happening. Aerated water off gases its C02 and I'm pretty sure this reaction frees up the carbonates in the tap water(which are ph adjusters Calcium carbonate) so the ph will rise....before the brew ingredients come together this reaction would happen fast and be very minimal at best as CO2 levels aren't high in tap water but then as the billions of microbes start to use up the oxygen to survive they are releasing CO2 to stay alive so the C02 reacts with the carbonates and the ph goes up. I'm just a humble electrician but from my understanding and how I too have experienced this same rising, this is my "crack-pot explanation!! The super tea gurus use collected rain water, clean running stream water, or pond water.....then last resort store bought spring water. Filtered water and bubbled tap can be too problematic but can still produce great teas.