Question What is causing random scrolling in ui elements (not 'UIElements') ?

This gif demonstrates it quite well, sometimes (often) there's this inexplicable behaviour where scrollbars just scroll up by themselves when dragged and highlights travel through hierarchies.
I don't have a gamecontroller attached, htc vive is the only separate input device, but steamvr is not running and the app is not using vr.

What could be causing this?