In my case, it was a combination of scapular instability, weak upper back, and way way too much pressing. I was young and dumb and benching 2-3x a week with horrendous form. I could feel the tendon slipping sensation most on on flyes and the bottom or BB bench. By the time the pain started was I dropping the benching down to 1x week, bringing the back work up, and complaining to my Dr who said it was bicepital tendinitis, then a labral tear, then rotator cuff tear, then slap tear...all without an MRI.
I finally said F it and went to an ortho, got an arthorgram/MRI 2 weeks later and the dye they injected all pooled in the stretched part of shoulder capsule. PT for 6 months, no improvement, surgery 2 months later, 6 weeks in a sling, 6 months PT, 6 more months PT at home with bands and broom sticks and light gym workouts.
My bench went from 300 to 135 and never really improved. My form is much much better but it still feels like my shoulder is being stretched.
Squating kills my shoulder (flexibility wise), but I was back to 315 in a few months
Front squats are not really a problem.
Deadlift went from 400 to 45, within 3 months 275 after a year 395.
I'm hesitant to really push the bench and DL for fear of re-injury, the mental part is much harder to overcome than the physical.
/hijack sorry for the long post.