Hmm (checks living room with a spirit level) . . .
I guess the sofa seat cushions are quite "bouffy" so they slope down towards the front. That said, I don't have too much of a problem personally, it's just as soon as the DDs go anywhere near the sofa then the cushions (not the sofa seat cushions, the extra ones) are on the floor in a flash.
Maybe it's a DD malfunction rather than a cushion malfunction!