I am one of those that had problems with the sound levels suddenly changing and It was hard to convince me that something was wrong other than the keyboard.
Doug is right in that there is a big interaction between the use of an expression pedal, the position of the APC slider, the position of the modulation wheel, how things are saved in the panel memories and, I might add, the items that have been selected in the Panel Memory Filter and the Data Protection Filter.
I can only explain it by the fact that the keyboard operating system "samples" all of these settings and produces a result based on the total aggregate.
As an example, the expression pedal, if used, does not control the audio output level directly like a volume control but it's value (depending upon physical position) is merely looked at by the operating system software and added to the mix. The same with the APC slider.
Under certain conditions these can all be altered by the panel memories and/or a loaded sequencer file.
Sometimes it's hard to find just where your little gremlin that's causing the problem is hiding, but it's there somewhere.
Keep looking and exploring and I am sure you will find and be able to correct the problem.
Good Luck,