Quote from the Bessen and Meurer paper (which is also the source for the $29 billion number):
Sixty-two percent of the time they feature software patents (Bessen et al. 2011) which are notoriously difficult to interpret. Allison et al. (2010) study patents litigated multiple times and usually asserted by NPEs; they find that software patents account for 94% of such lawsuits.