^^ These comments are wrong.
The osmotic processes of nutrient/mineral/waste exchange between the roots and the rez is very complex. As roots exchange salts and sugars and what-not, the pH will absolutely swing - sometimes wildly depending on what additives a grower has added and whether or not you have a sterile or living rez.
Perhaps a grower is using a bloom-boost with asorbic acid? Maybe there is a few ml/gallon of molasses? How about silica? What application rate for beneficials? What kind of oxygenation is provided to the system? Is it tap or RO water? Is the
cal-mag package calcium nitrate or calcium carbonate?
Anyway - drawing those definite conclusions from the bare-bones information the OP provided is kinda sloppy.
We have no idea why the pH is dropping.