There is a black tank vent in the toilet I'm not exactly sure it's purpose but I've read about them here on RV.net. You have the flush valve closed so the bowl is only half full. But the bathroom sink is full of water and water seeks it's own level. So the water slowly drains out of the sink drain and the water level in the tank rises to fill the toilet pedestal and then pushes it's way out the vent.
Look on the bright side, with how cold it is there you can probably use a snow shovel to just pick up the frozen spill off the ground and drop it in a dumpster.
Or throw it out in the road giving a new meaning to the term black ice :B :B
