From the Atlantic, a well-researched text on retractions:

by one estimate, from 2001 to 2010, the annual rate of retractions by academic journals increased by a factor of 11 (adjusting for increases in published literature, and excluding articles by repeat offenders) [2]. This surge raises an obvious question: Are retractions increasing because errors and other misdeeds are becoming more common, or because research is now scrutinized more closely?

The short answer: both.