I was told it a combination of a few things that cause it to rise. As the nutrients are consumed by the plants the solutions makeup fluctuates this causes a rise or fall. The bubbles and the introduction of air in the solution will cause a rise in ph every time.
After my first week of running RDWC I bought an automatic ph adjustment system. Only adjusts up or down. Mine has been set to down for the entire time. They are pricey but definately worth it. Those fluctuations can lock out those expensive nutrients. 5.8 everytime I look.