There are lots of factors at play so I cant tell you exactly your cause. But let me point this out to you before listing some. We almost always PH our nutrients down correct?
So if your soil PH was low then adding you PH down would not solve your issue. And it was infact grower error not ph adjustment.
Just because you feed at low doses does not mean you cannot have a buildup of nutrients and that usually causes low soil PH.
Ferts with ammonium nitrogen lower soil PH because the plants release h+ ions in the process used to uptake it.
Underwatering can also cause this... as you indicated. This is because as the soil dries nutrients become more concentrated and nutrients are generally acidic
Iron will also lower soil PH and to much cal mag thats usually is in nitrate form with iron added can also play a role.
The amount of bacteria respiration and air exchange capacity of the soil can affect PH.
Typically if you have a PH drop in your prebuffered soil such as
promix its nutrient related and if you have a rise in soil PH it usually water source related.