There is a whole ecosystem that is funding these lawsuits. It also operates somewhat anonymously, with funding sources working directly with law firms and indirectly with inventors (if they are even involved anymore). I've seen emails between law firms and the inventor that literally title the investors as "the funding source." The original inventor need not put any money at risk, and the law firms and funding sources do their own diligence to decide if it's worth proceeding to file suits. In most cases, the entity filing suit is created for each series of attacks so that if they actually lose a suit and are supposed to pay out, there are no assets from which to pay. Therefore, low risk and potentially very high reward.

The biggest factor for that is not the validity of the patents, but the breadth and ability of the patents to be filed against "juicy" targets with a lot of cash. The cost of filing a suit is literally in the hundreds of dollars (I think ~$750) and most of these cases get settled out fairly early because of the immense cost of the defendants to complete the discovery process.