One possible reason could be extremely high water flow. As carbon dioxide is dissolved in an aqueous solution, it undergoes hydrolysis, which yields carbonic acid. The higher the amount of co2
there is in water, the lower the ph. If there is an extreme amount of water flow, more carbon dioxide is exchanged with oxygen.
Lower amounts of co2
= higher ph
High amounts of co2
= lower ph
Btw, are you doing sufficient biweekly water changes? This is important as not doing water changes seems to increase the ph as well. The continuous increase of oxygen and decrease in carbon dioxide could also raise the ph in aquariums, lowering the acidity of the water. This effect is boosted when things such as limestone and crushed coral are introduced in the water; they raise the ph.
Sorry about the long explanation, I just like learning about chemistry, biological, and health sciences. Anyways good luck!