NFL : Biggest year to year declines 

                                             Change in
    All Time:                                 win %
  1 Tennessee Titans         1993 -> 1994    -0.625   (0.750 -> 0.125)
  1 Houston Texans           2012 -> 2013    -0.625   (0.750 -> 0.125)
  3 Chicago Bears            2001 -> 2002    -0.563   (0.813 -> 0.250)
  3 Carolina Panthers        2015 -> 2016    -0.563   (0.938 -> 0.375)
  5 Atlanta Falcons          2012 -> 2013    -0.562   (0.812 -> 0.250)
  5 Atlanta Falcons          1998 -> 1999    -0.562   (0.875 -> 0.313)
  7 San Francisco 49ers      1998 -> 1999    -0.500   (0.750 -> 0.250)
  7 New York Giants          2016 -> 2017    -0.500   (0.688 -> 0.188)
  7 New England Patriots     1980 -> 1981    -0.500   (0.625 -> 0.125)
  7 Indianapolis Colts       2010 -> 2011    -0.500   (0.625 -> 0.125)
  7 Denver Broncos           1998 -> 1999    -0.500   (0.875 -> 0.375)
  7 Dallas Cowboys           2014 -> 2015    -0.500   (0.750 -> 0.250)
  7 Baltimore Ravens         2006 -> 2007    -0.500   (0.813 -> 0.313)
 14 San Francisco 49ers      1981 -> 1982    -0.479   (0.812 -> 0.333)
 15 New York Giants          1986 -> 1987    -0.475   (0.875 -> 0.400)
 16 Tampa Bay Buccaneers     2005 -> 2006    -0.438   (0.688 -> 0.250)
 16 San Francisco 49ers      2019 -> 2020    -0.438   (0.813 -> 0.375)
 16 Philadelphia Eagles      2004 -> 2005    -0.438   (0.813 -> 0.375)
 16 New Orleans Saints       1979 -> 1980    -0.438   (0.500 -> 0.062)
 16 Las Vegas Raiders        2002 -> 2003    -0.438   (0.688 -> 0.250)
 16 Los Angeles Chargers     1999 -> 2000    -0.438   (0.500 -> 0.062)
 16 Green Bay Packers        2007 -> 2008    -0.438   (0.813 -> 0.375)
 16 Detroit Lions            2007 -> 2008    -0.438   (0.438 -> 0.000)
 16 Detroit Lions            2000 -> 2001    -0.438   (0.563 -> 0.125)
 16 Detroit Lions            1991 -> 1992    -0.438   (0.750 -> 0.312)

    Last season:
    San Francisco 49ers      2019 -> 2020    -0.438   (0.813 -> 0.375)
    Houston Texans           2019 -> 2020    -0.375   (0.625 -> 0.250)
    New York Jets            2019 -> 2020    -0.313   (0.438 -> 0.125)
    New England Patriots     2019 -> 2020    -0.312   (0.750 -> 0.438)
    Jacksonville Jaguars     2019 -> 2020    -0.312   (0.375 -> 0.063)
    Philadelphia Eagles      2019 -> 2020    -0.282   (0.563 -> 0.281)
    Atlanta Falcons          2019 -> 2020    -0.188   (0.438 -> 0.250)
    Minnesota Vikings        2019 -> 2020    -0.187   (0.625 -> 0.438)
    Baltimore Ravens         2019 -> 2020    -0.187   (0.875 -> 0.688)
    Denver Broncos           2019 -> 2020    -0.125   (0.438 -> 0.313)
    Dallas Cowboys           2019 -> 2020    -0.125   (0.500 -> 0.375)
    New Orleans Saints       2019 -> 2020    -0.063   (0.813 -> 0.750)