"Rapidly" is a relative term here. Even if millions of tons per year are evaporating away, that may be trivial. In fact, the earth alone has 330 million cubic miles of water, which is 1.5 quintillion tons. We could lose a 100 million tons of water every year for 10 billion years and still have some left over. Of course because of our distance and strong magnetic field, in reality we lose very little water or atmosphere.
This planet is much bigger, and although as a gas giant it wouldn't have oceans, it could still have as much or even far more total water as earth. I believe it is also thought to be younger than the earth, perhaps significantly, so even losing water at the huge rates listed above is not at all unreasonable. It is believed that gas giants can not form so close to a star, so the theory is that this one formed in a Jupiter-like orbit and migrated inward later on.
To figure out if this is reasonable, the next step (which some researchers probably have already undertaken) is to determine how much energy is required to evaporate this much water and heat it up enough to expand beyond the planet's gravitational reach. Then you compare that to the amount of heat believed to be absorbed by the planet from its star, based on the known distance and brightness of the star and the estimated diameter and reflectivity of the planet.
The entire atmosphere of this planet is gradually boiling away, and water is just one of the many compounds they've observed by spectroscopy.
http://antwrp.gsfc.nasa.gov/apod/ap070417.html"Any man whose errors take ten years to correct is quite a man." ~J. Robert Oppenheimer (speaking about Albert Einstein)
