[btilly]

The biggest factor is thought to be core-mantle coupling.

The Earth is built in layers. The solid bit we see is called the crust. Then there is a liquid layer where heat melts rock, that's called the mantle. And then at the center of the Earth the pressure again turns things solid, that's called the core.

All of the things affecting the Earth's rotation in the long term pull on the crust. The two biggest are tides (slows us by 2.3 ms/century) and glacial rebound (estimated to speed us up by 0.7 ms/century). The result is that the crust and the core wind up turning at different rates, and interact with each other through the mantle. Which transfers angular momentum back and forth between the crust and core.

The article that first showed this is https://www.nature.com/articles/333353a0 if you want to look further.