Condensation created by a slight leak in the seal of the freezer door and air bubbles in the ice that pop when the cubes reach a certain temp. They pop, in a frozen state. Or high levels of chlorine in your tap water at the time and that also may pop when a specific temperature is reached, causing the stalagmite creation, some type of chemical reaction. Just a theory not a fact, only things I can think of this early in the morning with lack of sleep!!
KEEP ON PUFFING!!!