IP Watchdog has an excellent breakdown. And this part of the analysis seems spot on:
Past litigation reports from Lex Machina have pointed to the fact that volatility in patent case filings are typically triggered by changes to the patent system, or even just proposed changes. Spikes in patent litigation have closely preceded changes like the abrogation of Form 18 to plead patent infringement in district court as well as the enactment of provisions of the America Invents Act. Given the fact that the debate on patent reform isn’t currently reverberating in Congress the way it has in recent years, it’s possible that the recent downturn in high-volume plaintiff filings is due to calmer waters in the patent system. The next foreseeable change to the U.S. patent system stem from the U.S. Supreme Court’s upcoming decision in TC Heartland v. Kraft Foods Group Brands, so it will be interesting to see if the court’s ruling in that case creates any similar volatility in case filings.