Well, let me give you a brief history.
First time, I seperated my left shoulder quite severely(2 years ago). Many things contributed to this, it didn't just happen.
1. I was overtraining training heavy without a break for over 4 months.
2. I had a vitamin deficiency, not a big deal but it contributed to the injury.
3. It was gray and rainy outside and I had to wait for my late training partner which made me lose my pump from warming up.
4. I was trying to add too much weight too fast. I added 5lbs on militaries. I was arching my back and going too low on the presses(something I thought I was suppose to do at the time)
5. My training partner distracted me at the 8th rep and I ended up dislocating my left shoulder.
6. I was using a smith machine.
I had slighly seperated my shoulders doing db presses and playing basketball earlier in my life.
I came back to training after depression and I built my delt strenght back up(very tough mentally and physically). I seperated my right shoulder in a very stupid move trying to do dips: the bars were to high.
I eventually got some physio done.
As you can see, I've analysed the shit out of this.
Hope it helps.