My understanding from someone who knows about this (please forgive my lack of knowledge as I am attempting to relay what they told me) was that they used to step down when going to land at fixed points to avoid turbulence and hold those levels.

The levels were picked to provide the customers a comfortable ride, based on reports from other planes and whatever other sources they have.

They have since been told to save on fuel and so don't do this, not "gliding" but lowering the thrust to save fuel when descending which exposes people to the rougher air for longer periods of time.

Please correct my understanding if this is not the case.