It is my understanding that you can blame it on the moon.
The same force that causes tides (but not "tidal" waves--which really have nothing to do with the lunar pull) are causing the Earth's rotation to slow down ever-so slightly. Eventually a day will be 25 hours long (In a few million years or so).
Maybe someone else can give us more scientific details.
I believe the Moon is also moving further and further from the Earth, Does anybody know: Will eventually escape the Earth more-or-less completely--and go into it's own orbit of the Sun thus becoming a planet?