DWC / Drip Feed. Flowering 3rd week Ph Lowering Const

Ok this isnt my first time growing, and I am very good a keeping an eye on everything in my room yet one problem every cycle happens and i havent been able to find the answer to it yet.

Heres some backround info for youall out there

Growing: NYC Desiel
I use a mixed system Drip Fed / Deep water culture
Keep PH :anywhere 5.2 allow to drift up to 5.9
PPM currenty 3rd week flower useing Foxfarm Nutes Tiger Bloom big bloom
800-1000 ppm

Now heres my question. So normally during the vegative stage i have to add PH down because my solution will always push up couple points a day.

Ive noticed with all grows that bout 2 weeks into flowering the PH steadys then starts reversing and goes down about a point a day.

Now can someone explain why this process happens

I understand that the plant changes and stops upward growth and starts to flower has alot to do with this change but exactly what is happening IDK

bloom nutes ive found for what ever reason are more acidic im in week five of flower and my change water with nutes is about 4 before i adjust to 5.8 but my drift is minimal and this is from ro water at 6.5

I have taken readings with both veg cycle nutes and bloom nutes and its about the same PH wise i dont think thats the case