Biggest declines from one season to the
next (based on regular season win percentage):
                                          Change in
       Franchise                            win %
  1 Oakland Athletics       1914 -> 1915     -36.8   (65.1  ->  28.3)
  2 Atlanta Braves          1934 -> 1935     -26.9   (51.7  ->  24.8)
  3 Miami Marlins           1997 -> 1998     -23.5   (56.8  ->  33.3)
  4 Cincinnati Reds         1981 -> 1982     -23.4   (61.1  ->  37.7)
  5 Cleveland Indians       1913 -> 1914     -23.3   (56.6  ->  33.3)
  6 Chicago White Sox       1920 -> 1921     -22.9   (63.2  ->  40.3)
  7 Minnesota Twins         1933 -> 1934     -21.7   (65.1  ->  43.4)
  8 Arizona Diamondbacks    2003 -> 2004     -20.4   (51.9  ->  31.5)
  9 San Francisco Giants    1942 -> 1943     -20.0   (55.9  ->  35.9)
 10 Boston Red Sox          1905 -> 1906     -19.5   (51.3  ->  31.8)
 11 Washington Nationals    1994 -> 1995     -19.1   (64.9  ->  45.8)
 11 Minnesota Twins         2010 -> 2011     -19.1   (58.0  ->  38.9)
 13 Chicago White Sox       1917 -> 1918     -18.9   (64.9  ->  46.0)
 14 St. Louis Cardinals     1931 -> 1932     -18.8   (65.6  ->  46.8)
 14 Oakland Athletics       1949 -> 1950     -18.8   (52.6  ->  33.8)
 16 Seattle Mariners        2003 -> 2004     -18.5   (57.4  ->  38.9)
 16 Los Angeles Dodgers     1991 -> 1992     -18.5   (57.4  ->  38.9)
 18 Cincinnati Reds         1944 -> 1945     -18.2   (57.8  ->  39.6)
 19 Detroit Tigers          1988 -> 1989     -17.9   (54.3  ->  36.4)
 20 Detroit Tigers          1919 -> 1920     -17.5   (57.1  ->  39.6)

    Last season:
    Baltimore Orioles       2017 -> 2018     -17.3   (46.3  ->  29.0)
    Kansas City Royals      2017 -> 2018     -13.6   (49.4  ->  35.8)
    Washington Nationals    2017 -> 2018      -9.3   (59.9  ->  50.6)
    Miami Marlins           2017 -> 2018      -8.4   (47.5  ->  39.1)
    Los Angeles Dodgers     2017 -> 2018      -7.8   (64.2  ->  56.4)
    Cleveland Indians       2017 -> 2018      -6.8   (63.0  ->  56.2)
    Arizona Diamondbacks    2017 -> 2018      -6.8   (57.4  ->  50.6)
    Texas Rangers           2017 -> 2018      -6.7   (48.1  ->  41.4)
    Minnesota Twins         2017 -> 2018      -4.4   (52.5  ->  48.1)
    Chicago White Sox       2017 -> 2018      -3.1   (41.4  ->  38.3)
    San Diego Padres        2017 -> 2018      -3.1   (43.8  ->  40.7)
    Toronto Blue Jays       2017 -> 2018      -1.8   (46.9  ->  45.1)
    Cincinnati Reds         2017 -> 2018      -0.6   (42.0  ->  41.4)