I had what you describe for years. To answer you’re question directly… I don’t know what is is. BUT, here are two things I did that made it stop for me.
I do 15 shoulder dislocates with my bath towel every night after my shower. I make sure to keep my scapula down and back throughout the entire movement and move slowly and carefully.
I dropped the standing overhead press from my routine and switched to the seated overhead press at 75 degree incline, with hands shoulder width apart, and making sure my elbows track out in front of me as I press rather than out to the side.
These two things made what you describe stop for me.