Mouse Keys randomly enabled. How can I prevent this?

I'm running macOS 10.12 on a MacBook Pro (Retina, 13-inch, Early 2015). Randomly, mouse keys was enabled when my hands were not touching the keyboard. I don't have a bluetooth keyboard (although I do have a bluetooth mouse which I was not using at the time). This is the second time it has happened. This is either a software or hardware bug, but it seems oddly specific to be a hardware bug when I don't see other random keypresses. 1. How can I diagnose this? I can't find any mention of 'mouse' in system.log. 2. Is this a known bug? I can't seem to find any other mentions. 3. Can I prevent this from happening again?