Aberration in the earth's rotational speed.

Factual Questions
1

Aberration in the earth’s rotational speed.

I am aware of the fact that the moon and tides are slowing the rotation of the earth. This is the reason for leap seconds.

http://blog.cloudera.com//wp-content/uploads/2012/07/leapsecond.png This is a graph of the earth’s rotational speed and leap seconds. Leap seconds occurred where there is a vertical spike in the line. When the line is sloping downward, the earth is rotating slower than it was in 1900. The steeper the line, the slower is the earth’s rotation. Between 1985 and 1999 there were nine leap seconds, one every 18 months or so. However, the next leap second was not added for seven years. What caused the rotation to increase?

What’s most unusual is that since 1998, there are visual aberrations in the graph from June to September. This is most pronounced during the summers of 2001-2005. During this period (when the line slope is upward), the earth’s rotation increased to the point that it was rotating faster than it was in 1900. What was the cause?

2

For an overview, you can check the Wikipedia leap second article:

Basically there are a number of more or less chaotic processes that affect the Earth’s rotation, including convective movements of the mantle, earthquakes which move large volumes of crust, and even human activities, mainly dam building and dam destruction which move large amounts of water around the surface.