Seems like it was due to a couple of factors:

1) Very old injuries, almost all when I was a child or teen playing sports or doing reckless things, which caused neuralgia (nerve pain). Two were on the head (one from skiing, one from playing baseball and getting hit with a bat on the head by accident). Another on the tailbone that am focusing on now

2) Very bad posture related to these injuries - had horrible posture as a teen, and thinking back, this is very probably due to my tailbone pain. It caused a ton of pain when sitting and also caused me to twist up my pelvis and hips to avoid it. This terrible twisting on top of the other pain destroyed my posture and clenched up my neck, and for the past 30 years, have been trying to undo those postural problems and muscle clenching. Have done a lot of physical therapy and exercises, but it's much better now (although, it's still a work in progress and will probably never be finished).