When the engine is started, the air compressor starts to build air pressure and your air springs will start to fill to proper ride height for travel. The dump button usually only is open as long as you hold it down, so the dump valves are closed. If the jacks have the coach level on unlevel ground, your ride height valves will try to fill or dump air to bring distance between the frame and axle to the proper setting. I wouldn't be concerned unless it continues to happen on level ground with jacks retracted.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI