Volume problem

Hi, I have a customer with an existing installation that I installed about 4 years ago, it consists of a matrix switcher and 4 zones each with 1 x 25 W amplifier. Sorce and volume functions are controlled via DLT switches. My customer has reported that several times the volume in some of the zones has increased to maximum seemingly by itself . In the short term I plan to return to the site and set the upper volume limits to a more tolerable level but does anyone have any clues as to why the volume ramps up by itself. Besides the individual DLT switches in each zone the only other place the volume control group address is used is as part of an "all off" scene which sets the volume in the zones to 0%.