ummm... my thoughts:
you set it backwards- instead of 1234, whatever numbers were on the opposite side were set.
you didnt do something and so when you spun the wheels to numbers then they clicked into place. (although for it to spin to 1313 by coincidence is bizarre)
someone is messing with you.
personally, id say my first thoughts.
good luck. and you might consider getting a lock exorcist.
