I was driving a relay from Arduino Uno R3 Pin 13. I observed that every time I upload some code the led on the board designating pin 13 would blink. I'm curious why this is so. I understand that the serial LED blinks cause there's a serial transmission but why an IO pin?
See What happens when code is uploaded using the bootloader? - the flashing is by design.
Probably the reason is to confirm that the bootloader has started.