I have a ~3 gal batch of a traditional mead. I have racked and watched the airlock activity go from nearly constant bubbles to no activity at all over a period of 2 months.

About a month ago I decided to remove the airlock and place a rubber stopper in the carboy for bulk aging. Everything was going fine, my sg was measured at 0.999. The rubber stopper stayed in place for about 2 weeks. Until, I was sitting in the kitchen last week and heard "POP!". I looked around to find my rubber stopper rolling across the floor. Now the stopper will only stay on for about 1 day then it will shoot itself across the room.

Is this normal

The mead is a little hazy with a 1/2" bed of bright white lees on the bottom and a small amount of lees on the sides of the carboy.