How can the Earth's rotation speed up? It is known as a fact that Earth's rotation is slowing every year, that's why we sometimes have 61 seconds long minutes.

In the Java documentation of Time : http://docs.oracle.com/javase/7/docs/api/java/util/Date.html , it is said :


  Some computer standards are defined in terms of Greenwich mean time
  (GMT), which is equivalent to universal time (UT). GMT is the "civil"
  name for the standard; UT is the "scientific" name for the same
  standard. The distinction between UTC and UT is that UTC is based on
  an atomic clock and UT is based on astronomical observations, which
  for all practical purposes is an invisibly fine hair to split. Because
  the earth's rotation is not uniform (it slows down and speeds up in
  complicated ways), UT does not always flow uniformly. Leap seconds are
  introduced as needed into UTC so as to keep UTC within 0.9 seconds of
  UT1, which is a version of UT with certain corrections applied. There
  are other time and date systems as well; for example, the time scale
  used by the satellite-based global positioning system (GPS) is
  synchronized to UTC but is not adjusted for leap seconds.  


Why is that so? How can this occur?
Any body with an irregular mass distribution will experience those wobblings. And more if it is not just irregular but fluid.