I'm not sure about individual cases, but I think I might have some insight into why there is a huge backlog.
I was a FTE and then contractor for Keypoint. I'm a conscientious and hard worker and was able to knock out a lot of work. Sometime around 2013-14 the company changed drastically and became obsessed nickel and diming workers. They cut out the car program, effectively did away with the bonus program, and started cutting corners any way they could. They shortened the artificial deadlines known as ACDs and decreased the flexibility in pushing back this date. Meanwhile they became more draconian in enforcing all this and increased pressure on investigators. As a result, as a contractor, I simply reduced the amount or work I did (by ~70%). And when I got crap about not making an ACD a few times (b/c a Source or Subject was sick), I, like other experienced and hardworking investigators, just threw in the towel.
Keypoint doesn't give a sh*t and I've even heard some management people threaten investigators about performance by bragging that they don't have a problem filling the seats in training each month. Anyone who has worked in this field knows that it takes two years in the job before an investigator knows the job. This is common wisdom I heard from numerous people. So the product of inexperienced investigators and the culture of threats has no doubt destroyed both quality and production. Could you imagine building houses with workers who've only spent a couple weeks in a classroom and a couple weeks shadowing or being shadowed? But even with the barebones checklist OPM investigations, which are kind of like requiring only a door, a roof, and a couple of windows, with totally inexperienced workers you'll have problems making even the house stand up on its own.