It looks like discovery is the cause - quoting from [0] "After all, the great struggle in every patent litigation is balancing the fact that discovery is by far the most expensive part of the process...".

It would seem that basically patent trolling is really discovery trolling - pay us or else we will make you go through discovery.

[0] http://patentlawcenter.pli.edu/wp-content/uploads/2010/09/Di...