The main effects are the torques applied by the Sun, Moon and a few planets. The rest comes from the movement of air masses, the resettlement of the crust after an earthquake, the rebounding of the crust from the last Ice Age and the peculiar way humans are distributing trillions of gallons of fresh water. At the level of a change in the Length Of Day by a microsecond per year, measurable with atomic clocks, there is quite a lot of movement going on. Visit the US Naval Observatory National Earth Orientation Service for more details.