In terms of OPS differential, the Cubs are having an historically large year over year decline

Last year, the Cubs had the 3rd best OPS differential since 1914 and best since 1939. See Cubs finish tied for 3rd best OPS differential since 1914 & tied for best since 1939. But so far this year, it is -.015 (.715 - .730).

I looked at the top 10 teams before last year in OPS differential (your team hitting OPS minus the OPS your pitchers allow) and how they did the following year. The lowest of the "next year" OPS differential for any of the previous top 10 teams was .043 (the 1940 Yankees). So none of them ended up close to negative.

The Cubs still have time to turn things around. But last year they were +.139. So their swing or decline is .154, much higher than any of the previous top 10. The biggest drop among them was .115 for the 1939-1940 Yankees. The drop of .154 would be the third biggest drop of ANY team from 1914-2014. The only teams worse are the 1915 A's, whose decline was .186 and the 1998 Marlins, whose decline was .157. And both of those teams lost many of their good players. The A's owner, Connie Mack sold some of his best players (if I recall correctly, it was because they got swept by the underdog Braves the World Series in 1914). The Marlins won the series in 1997 but did not want to pay to keep many of their good players in 1998.

Here is how those 10 did the next year

Team	Year	OPS DIFF	Next year	Decline
NYY	1927	0.196	0.094	0.102
NYY	1939	0.158	0.043	0.115
ATL	1998	0.139	0.086	0.053
BAL	1969	0.136	0.080	0.056
NYY	1936	0.131	0.121	0.010
STL	1944	0.130	0.044	0.086
STL	1942	0.127	0.114	0.013
CLE	1948	0.127	0.046	0.081
NYY	1998	0.126	0.089	0.037
SEA	2001	0.126	0.044	0.082