Interesting...If I were to guess "why"...I'd say the core(s) of the earth are getting less dense. Clearly you can't make matter...and I doubt that much junk crashed into the planet over such a 'short' period of time...so the mantle must be expanding -- that's actually pretty cool.
The water? There's tons of crap (gases, too) hidden in the mantle. If it becomes less dense, it will release that stuff -- most of it was probably hydrogen and oxygen to begin with. Which when combined, we all know makes water. (Not to mention the ice-balls hitting the planet over those millions and billions of years)
It stops, I hope? I'd hate for the planet to expand till we're just a cloud of debris....
