The most common category is "primary spontaneous". Predominantly young (85% <40yo), healthy males (6 times more common). Smoking contributes, and I believe being tall and skinny does so as well.
Short and fat--never a smoker. Still collapsed my right one and then the left one a week later. The doctors said they'd never seen that (bilateral in such a short timespan with no apparent cause) and had no idea what could be causing it. I had to laugh. Pectoris Excavatus or other skeletal abnormalities can cause it but I don't have any of those either. Just luck of the draw I guess.
Sometimes it feels like a time bomb. I've been living with it for so long I just don't care anymore, it actually makes no difference in my life or plans. It prevented me from becoming a military pilot though.