I think it's global warming. It has to be.
Heavier material in the mantle subsiding towards the core could do it just as easily, and there is a lot more mass beneath the crust than above it.
The problem is that the atmosphere is so light compared to the rest of the earth that even at the nanosecond level I have doubts you would see this effect. To affect the rotation of the Earth by whole seconds/year you need to be talking about yottatons of rock shifting.
And wasn't lately not also something with the magnetic field?
> They also have begun wondering if global warming might push the Earth to spin faster as the snow caps and high-altitude snows begin disappearing.