Good question. It means that something is moving mass away from the axis of the Earth.
There are only two candidates I can think of:
1. Movements in magma below the Earth's crust.
2. Global warming. Less ice at the poles means that water has moved away from the Earth's axis, which will slow the rotation down. I haven't run the maths to see if that adds up though.
There are only two candidates I can think of:
1. Movements in magma below the Earth's crust.
2. Global warming. Less ice at the poles means that water has moved away from the Earth's axis, which will slow the rotation down. I haven't run the maths to see if that adds up though.