One of the studios i work in has Apogee Ensemble audio interface - and same thing occurs. Not only when Cubase is opened, but also whenever in/out ports are reassigned and whenever audio buffer size is changed. Really scary stuff. Almost burned my Genelecs once…
Looks like it is a Apogee=Cubase issue. I searched a lot and found no way to fix this. So lowering the volume on your monitors and always working close to maximum system volume would be a workaround.