It's something to do with the frame rate of the video lining up with the vibration of the ruler. I don't know if the frame rate changes in the light or something..
It's a single continuous shot, so there was no shutter speed increase.
You're seeing the effect of increased light, therefore smaller aperture, therefore deeper depth of field, and yes, as above, (enough extra light for) frame rate effect to be visible.
You're seeing the effect of increased light, therefore smaller aperture, therefore deeper depth of field, and yes, as above, (enough extra light for) frame rate effect to be visible.