–
The LEDs are powered by a 50 Hz AC supply. Since LEDs are diodes, they light up only during the positive half cycles of the AC supply, but do not light up during the negative half cycles.
This flashing of the LEDs at 50 Hz is revealed if we keep moving the LEDs, as shown in this video.
xmPhysics 
One thought on “Why do LEDs leave a trail of dashes?”