https://scripps.ucsd.edu/news/global-warming-influencing-global-timekeeping
Why did this happen? Several things cause the rotation rate to vary from year to year. Some change the rate by the same amount every year, but two phenomena are changing Earth’s rotation rate by different amounts each year.
The first phenomenon is the melting of landice at high latitudes. Much more has been melting as the polar regions have gotten warmer, one sign of global warming. The meltwater goes into the ocean and raises sea level over most of the ocean. This amounts to a transfer of mass away from the poles towards the equator, which slows down the Earth’s rotation rate. Measurements of Earth’s gravity show that before 1990, gravity changed in a way that would make the Earth closer to a sphere, and made it rotate faster. But since 1990, this trend has reversed and made the Earth rotate more slowly. Global warming, by moving water from the poles to the equator, has changed how fast the whole of the solid earth is spinning.