The Biggest Declines In Winning Percentage For Pennant Winners, 1901-68

The first table has the AL. W-L%1 is a team's pct. the year they won the pennant and W-L%2 is their pct. the year after. All data from Baseball Reference and Stathead.

The 1914 A's had a pct. of .651 and in 1915 it was .283, for a change of -.368

Tm	Years	W-L%1	W-L%2	Diff
A's	1914-15	0.651	0.283	-0.368
Twins	1933-14	0.651	0.434	-0.217
White Sox	1917-18	0.649	0.460	-0.189
Red Sox	1912-13	0.691	0.527	-0.164
Red Sox	1946-47	0.675	0.539	-0.136
Yankees	1964-65	0.611	0.475	-0.136
Orioles	1966-67	0.606	0.472	-0.134
Yankees	1939-40	0.702	0.571	-0.131
Indians	1954-55	0.721	0.604	-0.117
Red Sox	1918-19	0.595	0.482	-0.113

Now the NL.

Tm	Year	W-L%1	W-L%2	Diff
Cards	1931-32	0.656	0.468	0.188
Pirates	1909-10	0.724	0.562	0.162
Reds	1919-20	0.686	0.536	0.150
Dodgers	1916-17	0.610	0.464	0.146
Dodgers	1966-67	0.586	0.451	0.135
Pirates	1960-61	0.617	0.487	0.130
Giants	1913-14	0.664	0.545	0.119
Phillies	1950-51	0.591	0.474	0.117
Dodgers	1963-64	0.611	0.494	0.117
Cubs	1918-19	0.651	0.536	0.115

The 1914-15 A's are the biggest decliners. But that is because Connie Mack sold or traded away some of his best players. Some had jumped to the Federal League and Mack did not want to get into a bidding war.

Important players who were gone in 1915 include Eddie Collins and Frank Baker. SS Jack Barry played less than half the season with the A's. Star pitchers Eddie Plank and Chief Bender were also gone. See SABR bio of Connie Mack by Doug Skipper. 

Many other players and pitchers from 1914 were still there. But their team OPS+ fell from 114 to 87 (114 means you are 14% better than average, 87 means you are 13% below average). Their team ERA+ fell from 94 to 68 (they had been giving up 6% more runs than average and it went to  32 more than average). The + sign also means the numbers are park adjusted.

The next biggest fall is the 1933-34 Senators. A quick look at them shows that their personnel did not change much and did not look especially old in 1934. Maybe I will do a future post to figure out what happened to them.