its because someone left the bottom drawer open more than about 5 minutes. same thing happened to me, and I had a tach out when it filled up with frost and he told me the water line gets affected or clogged somehow when the bottom drawer is left open very long. In my case, I was cleaning the fridge and left the drawer open to do so. He said the entire cooling of the upper part of the fridge eminates from the bottom freezer part. Interesting.