Then I heard a bang.
Eep?
T'was one of the narrow cd bookshelves, which I've been meaning to shim (so it was empty) - it fell over with a bang! At least it landed partially on my laundry, so it wasn't quite as noisy as it could have been.
There must have been some earthquakes recently, because the other shelf was also moved away from the wall. Nothing big, just cumulative effect?