In my system that is quite normal during large growth spells. I see my largest PH drop from week 2-4 in flower why they are getting tall, then it calms down again till about week 6 when I start to notice the bud filling out. I think it happens when the plants are in big development phases, all of my harvest have done the same thing, I even posted the same question 6 months ago.... Seems normal, I get good yeild, color, and high.