That's also my theory: Country houses used to be partially shut down, a whole wing, which may only be required as an accommodation for guests, or the household may contract to a manageable core of room in winter. Every now and then, something from these shutdown rooms would be required and somebody would be sent out into this eerie realm of dust covers. A faint rustle in the dark, there, and again (a disturbed mouse), breaking the silence of these deserted (and supposedly quiet) rooms, could make this quite the challenge for the faint hearted. And what ultimate of psychological horrors could be lurking in the dark, other than these covers coming to life?

However, there's another theory about this (not mine, sadly): "The most common ghosts are the ones that appear dressed in a white sheet with flailing arms. These are people who died while changing their duvet covers and they are condemned to wander in bedrooms forever, trying to find the corners." ;-)