I found a helpful link listing the 10 biggest comebacks to win the pennant which details some of the biggest turnarounds, but not necessarily the reason for the reversal. The biggest listed here seems to be the 1914 "Miracle" Braves.
The Miracle Braves were 15 games out of first place on July 6 but went on to go 68-19 in the final half of the season and won the pennant by 10 games.
The Wikipedia page goes into more details.
The Braves performed one of the most memorable reversals in major league history, going from last place to first place in two months, becoming the first team to win a pennant after being in last place on the Fourth of July. After finishing in fifth place in 1913 with a record of 69 wins and 82 losses, the Braves were not expected to be contenders. They spent the first part of the season in last place, posting a record of 26 wins and 40 losses in early July. Led by three pitchers, Dick Rudolph, Bill James, and Lefty Tyler, the team began to win games, taking over first place for good on September 8. Their record over their final 87 games was 68–19 for a winning percentage of .782. Infielders Johnny Evers and Rabbit Maranville led the league in double plays. The Braves went on to sweep Connie Mack's heavily favored Athletics in four games in the 1914 World Series. The team became known as the "Miracle" Braves and remain one of the most storied comeback teams in baseball history.