Heck you have a Glock 21sf as a BUG? What is your primary?
I carry a BUG in my front left pocket and I have caught several times if I have bumped into a counter/table that sometimes the mag-release has been pressed. In my case being right handed the solution is to carry it in my right pocket so the mag release button doesn't get hit if I bump into something. So I don't know if your case is really anything Glock specific or simply a normal use problem with mag releases.