How can a chandelier rotate on its own?

At night a few times I have noticed that the chandelier in my house rotates 90 degrees back and and forth. It will do this for a few minutes at a time. How is this possible? It hangs from a chain connected to the ceiling, but I don't see how it can move without anyone else touching it. I spend much time under the chandelier, and I've only seen this happen at night. How is this possible?