From the WSJ:
In a statement, the Redmond, Wash., software company said Mr. Ballmer sold the shares for financial diversification and “to assist in tax planning” before the end of the calendar year
With long term capital gains rates scheduled to go up sharply Jan 1, it was only prudent for the sale to be made this year.
Just imagine, however, how few and far between sales of this sort will become next year if tax rates on the wealthy are allowed to jump. A fearless prediction: although rates may go up, tax collections will go down.