It sounds like you are engaging a carved turn during a skidded turn without actually trying to do it. Learn to harness this skill and the board as it is the "ideal" way to make turns and is easier on the legs.
Just to make sure... do you normally make skidded turns where you hear a scraping noise? Then this "motion" happens and you feel the "second edge" which feels really smooth and fast, similar to gliding on an ice skate? If this is how it feels, then you are simply starting to carve your turns instead of skidding... good for you!
Another good way to tell if you are carving turns is to listen to the sound of the board on the snow. If you hear scraping noise, like trying to put a tiny bit of butter on really dry toast, then you are doing a skidded turn. A carved turn is much more quiet and this skidding noise should be significantly reduced. Try to listen next time!
Another way to tell is to look at your tracks when riding on hardpack snow. Tracks from a skidded turn are pretty wide, anywhere from a foot (30 cm) to a few feet wide, depending on board size and how much you are skidding. WHen you are carving, the track is really thin because only the edge is coming in contact with the snow, so you dig a deep, thin line in the snow.