Why do rabbits keep turning their big sound-collecting ears?

They are listening all around.

The animal is listening to the sounds around itself and other rabbits around the warren. The animals need to keep moving their ears around as there are so many noises around them, they are a prey animal and so always have to keep an ear out for the predators which may be aproaching them from any angle. This constant moving of the ears enables the animals to listen for any sound which may become a threat to itself or the other rabbits in its warren (as rabbits tend to be social creatures and graze together), if the animals did not use their Excellent hearing all the time the animal would not live very long.