This is a board with 80 3535 LEDs on it just after completing the reflow cycle in my home-made reflow oven. Why are the bottom corner LEDs lit up? Rather, how is this happening?
I'm going to guess that it has to do with the thermocouple I see touching the board vs. the metal insides of the oven (which are touching the bottom of the PCB).
Insulation conductivity increases exponentially with temperature so a small amount of leakage at room temperature can become quite significant at red heat.
It probably indicates that your oven interior is not properly earthed.