Weird behaviour from keyboard and various software

Running Windows 10 on a desktop. Replicated with different keyboards. All of this happened yesterday

1. Shift key in Firefox causes LEAVE PAGE. But no effect in OpenOffice

2. Typing an entry into File Explorer tries to autocomplete but flashes the title and won't allow deleting characters unless Shift Key is held down
3. Hovering over the folder list in File Explorer causes scroll down which stops if the shift key is held down. On the body causes scroll left

4. Same behaviour on Start Screen
5. Holding the Ctrl key without a plus sign causes zoom in office

6. All drop down menus in FIREFOX and other apps appear for an instant then vanish and can't be accessed
Help please

I had tried different keyboards and a different mouse and tried them in different USB ports. I also did a soft reboot. Nothing worked.
Then I unplugged all the USB devices and plugged them back in one at a time and it's all fixed - but I've no idea why.