The reason is that tiny organisms are decomposing and falling down from the layer above. With plankton growth and shad feeding on the plankton, most bass are going to be just under them. I like to fish from the thermocline and up about five feet. This is the zone I target. Don't get me wrong, I also fish shallow, but it's usually early or late in the day.
Wind can also change the thermocline.
I don't worry too much about it unless there are strong winds for a couple of days. Some lakes won't have a thermocline. The level of thermocline usually follows clarity. The clearer the water, the deeper the thermocline. On dingier lakes, it may be only six or eight feet.