I thought that hotels can apply minimum stay requirements as and when they pleased? I've certainly experiences similar at other properties for both reward and paid stays.

Obviously I'm not a lurker, but until they arrive, that's my experience of it anyway.

Not sure why the hotel has that weekend on that basis, unless the cake show at the Excel centre has a lot of people staying over for it. May be that the organisers have blocked out a large volume of rooms up front, so the restrictions may drop nearer the time.