I don't know if it applies, but some drum synths and other plugin instruments for some reason I have never understood - connect up their Master Volume to CC 7, which then causes the Master Volume to get changed when it is neither expected nor desired.
I use Batter 3 and Kontakt 5, both from Native Instruments, and I go into the options for each of these synths and change the defaults so that neither synth connects their Master Volume to CC 7 changes.
For ME, the above clears up that issue - perhaps you could list which synths this is happening to you with, and then maybe we could help you get it figured out and resolved.
Bob Bone