I don't know about ALL ferts but I know for sure that the Fox Farm nutes/fert I use do cause the pH to go down. I've been told my a hydro store that "depending on what you're growing" (they always say that since its illegal to grow in my state), that plants will generally change the pH themselves as they grow as well. So even if it isn't your nutes or fertilizer that is causing the pH fluctuation, the soil might start getting concentrations of certain things that will also change the pH. So it can be a number of things which is why it is always a good idea to check the pH of water going in or out.