Buildup of nutrients can cause a drop in ph among other things like microbe respiration or death. Especially nutrients with a higher ammonium source of nitrogen.
Ph of water in makes little difference unless using a pretty hard water.
The soil composition and buffering capacity play a huge role and so do the nutrients used and concentration as well as microbes.
If using a decent water source in soil I would be far more concerned about the ph in the media rather than the ph of your nutrient solution.
It's covers more than you need but will help you understand the how ph works... its all about alkalinity.
In this article, we discuss proper pH levels for your weed plants; alkalinity, soil pH, tap water, and the effects of nutrients on pH when growing marijuana