Like CC, I was also one until I was about 13 (I am 43 now). Do not know why I did it or why I stopped. It just happened.
I know I used, and still do, to go into deep sleep rather fast. Not necessarily REM, but so deep that you could drive a truck through my bedroom and not wake me up. My accidents normally happened within the first hour I was sleeping, in spite of going to the bathroom minutes before falling sleep.
I was embarrased. My friends knew because my parents used to put my matress outside to dry and vent out.
There was nothing I could do.
Good luck!