Bummer! You can find a few other posts here. I recall one where they wrote that the back pulled out when they removed the watch from the magnetic charging puck. I think they were sport models.
Out of curiosity does any of the following apply to your situation? Excessive sweating, swimming with the watch, application of lotions, sunscreens, etc. on the wrist where you wear your watch, wearing the watch while working with solvents of any kind (including getting the watch wet with dishwashing suds), unusually harsh operating environments that are either high heat and humidity, unusually cold, etc. ?
I'm just wondering if it's a manufacturing defect in some watches, or if something is dissolving the seals on the back of the watch...
I have definitely sweated a lot in mine since I have owned it, and cleaned it off with a soft cloth damp with water. I'm glad I have AppleCare, although this should be something that is covered regardless of whether or not you purchased additional protection. Please let us know.