If a therapist, like me,  jumps on your back to give it a crack, it may be that they are acting out a  “meniscoid entrapment theory” moment. This  means, they are speculating that vertebral joints have small menisci  (synovial folds) that gets trapped within the joint. This entrapment causes  guarding  and preventing movements ( that group of facet joints lock).
The idea of using a manipulative techniques  is, so some think, is to  change the pressure within the joint , pushing the synovial  folds back out.
Originally proposed by Kos and Wolf. elaborated by Bogdvk And Jull.
Frankly, I think my theory that your back is invaded by evil demons, makes far more sense!