What element of physics is at work when...?

someone pulls the chain to a pull-chain light bulb (the one's that hang from the ceiling in garages and storage facilities) and the light bulb begins to sway. In other words, when I pull the chain to the light bulb to turn it on and then let it go, more often than not, the chain and the bulb it's attached to begin to sway subtly back and forth. What element in physics makes that happen?