The simple answer to 'why?' is that the site is very busy. As the content is database driven, that has to be requested and returned to each user. At times there may be too many for the server to handle so some get dropped or you get a partial response.
Some stats to spice up this dull subject:
December 2010 - 3.7m
December 2009 - 1.9m
December 2008 - 1.7m
December 2007 - 0.87m
December 2006 - 0.65m
December 2005 - 0.42m
December 2004 - 0.2m
Those are the number of pages the site has returned for the month of December each year.