attached is a graph of corporate bond yield and t bill for usa from 2000 to current. i want various reasons for the fluctuation that happened .

regards

FRED 10 2000 Sources: Moody's, OECD fred stlouisfed.org Moody's Seasoned Baa Corporate Bond Yield@@ Long-Term Government Bond Yields: 10-year: Main (Including Benchmark) for the United StatesO 2004 2008 2014 2002 2006 2016 2010 2012 myf.red/g/diR9