More on US patent litigation statistics - correlation with granted patents

Once a Kat always a Kat.  This moggy was delighted to receive an observation from Emeritus Kat Mark Schweizer (now a Rechtsanwalt in the Swiss firm of Meyerlustenberger Lachenal ) in response to the post a few days ago about US patent litigation statistics.

Says Mark:

Regarding your post - I was always wondering how the increase in litigation compared to the increase in patents granted. Turns out, the study answers this, with a quite striking graph on page 6. There is an almost perfect correlation between patents granted and patents litigated. 

Which seems to indicate, to this naive mind, that not much has changed with regards to the decision whether to litigate a patent or not. A small, steady, fraction of granted patents are litigated. That seems to indicate that neither patent quality nor incentives to sue have changed dramatically between 1991 and 2012. It is simply that more granted patents lead to more litigation, which is not exactly surprising. And which is not the public perception, with the brouhaha over NPEs and such.

The graph is this one, which even this untutored moggy can see shows remarkable correlation.

The IPKat of course fully agrees with himself.  How could he not?  But Merpel recalls from her school applied mathematics lessons that in the realm of statistics correlation does not imply causation.  So, is it the existence of the patents that drives the litigation, or does the desire to conduct patent litigation drive the obtaining of patents?  Or are the numbers of patents and amount of patent litigation driven separately by other considerations, which happen to correlate well?  What do readers think is the correct explanation for this extraordinary correlation?