Fordham has a new study on smartphone patents and their assertion. Here is the money quote:
The most frequent plaintiff in the dataset [is] Apple. Apple has a uniquely aggressive litigation history when compared to the rest of the market leaders and Apple's stance may have effected a more general increase in litigation within the market. First, Apple has filed more lawsuits than other market share leaders. Second, Apple began asserting its design patents related to its smartphone in 2011 and prior to these lawsuits, design patent suits were vary rare in the market. Finally, patent infringement litigation saw a substantial increase after Apple's broad patent litigation was filed in 2011. Apple's aggressive litigation posture may be spurring litigation throughout the market and may be motivating competitors to acquire additional patents in order help them strengthen their defensive position.
See THE IMPACT OF THE ACQUISITION AND USE OF PATENTS ON THE SMARTPHONE INDUSTRY (WIPO under a Special Services Agreement) at /media/docs/2013/08/clip_study.pdf.