My guess is that the failure point is the rubber elbow. Once water gets inside it rots the wood and causes it to pop loose at the seams.

This is what the Sonance subs installation manual says....

"When placing the rubber elbow onto the subwoofer, it is recommended to add General Purpose Marine Grease to the inside of the elbow. This grease is available under the brand name Sta-Lube. This lube will act like a water barrier and ensure that water won’t enter in through the elbow connection."

The Episode manual doesn't mention this step.