I wouldn't be surprised to learn that several players need some rest to heal up from the long season and in preparation for the playoffs, and that need is coming at a time when they are trying to clinch the division.
My first thought is, why is this need for rest affecting the Cardinals more than the teams they're facing?
I think it's because the offense isn't nearly as balanced as the Cardinals hoped. Goldschmidt and Arenado are having MVP years, but they've been dragging. Others aren't stepping up.