Here's three different stories why the Earth is slowing it’s rotation. Perhaps someone is using the dart method.

One. - They say increasing levels of carbon dioxide (CO2) in the atmosphere will slow the Earth's rotation. (February 12, 2002)

Two. - "As the outer core turns, its magnetic field is deflected by the patches and the core material gets slowed down, just like mountains rubbing against the atmosphere slows the Earth down," he added. (February 21, 2001)

Three. - Our planet is not a perfect sphere. Its slightly squashed shape, combined with the gravitational effects of the Sun and Moon means the Earth's rotational speed is changing. (December 30, 1998)