Two main things to think about,
1) Photoperiod - less daylight hours has affect on most of God's creatures. Bass, and all of nature, react to the subtle changes in amount of sunlight seen over time as fall and winter approach.
2) We all tend to think about baitfish, however even more so we need to consider the food source of the baitfish ----- phytoplankton ( the green coloration of water is caused by these microscopic organisms ), then zooplankton, primary food source of shad. Since phytoplankton is consumed by zooplankton and phytoplanton populations drop as the amount of sunlight decreases, the shad migrate to back of creeks late in fall in search of more futile water which still has the inflow of nutrients aids the population of phytoplankton. Follow the food chain, current, wave action move these microscopic organisms in the water, shad move in to feed and the bass follow.