And why does the tank leak when it's low? I thought it would be reverse.
Physics.
The tanks work by creating a light vacuum inside the tank. The light pull of the vacuum keeps the juice inside the tank rather than all leaking out through the wick. This vacuum is kept in place with a pressure differential between the outside air an the air inside the tank. The juice itself provides a sealing effect by pressing against the outside air and using surface tension to hold the seal.
Now, when your tank is low the liquid looses the weight and pressure it needs to maintain that seal. There isn't enough weight of the liquid pushing down on the seal to hold the surface tension. This allows air to bubble in to the tank which breaks the seal, allowing the liquid to run out.
An easy analogy would the floating valve in your toilet. With the top tank full of water the valve is pressed closed and the water stays in the tank. However, after you flush the toilet and most of the water has been emptied from the tank the valve will float on it's own and water will continue to drain out even though you're not holding the handle anymore. The valve doesn't recreate the seal until enough water has been reintroduced to the tank to press it back down into place.