Once you wrapped the line around the spine of the biner you no longer had a Munter hitch; it may have still worked on rappell to some degree but it was not a Munter hitch once you added the wraps.

http://www.chockstone.org/TechTips/MunterHitch.htm

Glad you didn't get hurt when you fell. Some sort of friction backup either in front or behind of your belay device could prevent that.