Gradual. When my T8-T9 popped, I was doing pull-ups of all things, so there was no compression which means it could have been anything preceding that, but I suspect years of military/shoulder press or squats contributed.

I did dead-lifts several years prior to the injuries and stopped those when I had a funny "clammy skin" sensation in one of my arms; it freaked me out a little and I hated deadlifts anyway so I always thought it was just my excuse not to do them, but in retrospect I think that was probably a wise decision.

My guess is that it could be deadlifts.