Was it uncommon for the 30 year swap/Treasury spread to be negative from October 23, 2008 to the present? (few example dates each year to compare rates - say June and December of each year). What causes this?

swap rates here: http://www.federalreserve.gov/releases/h15/data.htm

Treasury rates here: http://www.treasury.gov/resource-center/data-chart-center/interest-rates/Pages/Historic-LongTerm-Rate-Data-Visualization.aspx ( the dates can be changed to get other periods).