It's super curious it even happens at all though, because the magnet keeping the charging puck stuck to the Watch really is quite weak. I'd be surprised if it could resist anything stronger than wet tissue paper actually, much less be able to suck the backpiece off...
I haven't seen any of the other cases mention if their watches were say, unusually warm for example; heat often losens these types of adhesives. That's the only thing I can think of that might be a cause for this to happen. I haven't really checked this out carefully myself, but when I've put my Watch on charging it doesn't seem to become particularly warm. However maybe this isn't true for everyone.