Volume jumps - why - alarming problem

Something has gone seriously wrong with my set - up (Cubase Pro 10.0.4 and MOTU 8pre interface on a 27" iMac.)

The header bar in the computer has a drop down box with the option to use internal speakers or the MOTU I choose the MOTU. The volume control for this is normally set at about 25%. For no apparent reason when working with Cubase it will suddenly jump to Maximum - causing me to have to dive for the stop control before the speakers blow or I go deaf or both.

I have looked at various controls for the volume which seem to be all over the place in mac software - but I can’t get back the reliable stability I had before and am scared of monitoring at more than a whisper for fear of this random change happening causing damage.

Grateful if anyone has an idea about how to fix this.

OK - I’ve done some more research and think I have identified what triggers the problem.

If I go to add an instrument track - (external instrument, my Kawai piano) - and check the volume slider it has jumped to maximum. I can then turn the volume down. If I then remove the added track and check again the volume has jumped back up to maximum.

Weird. Any ideas?

Perhaps an automation node has been added accidentally.

Thank you - but it’s not that.

Same problem occurred just now when I added an audio track for microphone.