I'm with Duke Dive Medicine (and, btw, I am NOT a cardiac surgeon. I WAS a general surgeon, but I'm an ER doc nowadays).
The cause of spontaneous pneumothorax is abnormality of the underlying lung. The lung can have blebs, or blisters on its surface, that are fragile and prone to popping. Or it can have areas of emphysema (abnormal lung tissue) that are less flexible (fibrotic). We know there is a significant recurrence risk for spontaneous pneumothorax, and it is higher with each recurrence. Did you end up having a surgical procedure to tack the lung up (pleurodesis)?
It's disappointing news, I know . . . but as we say about diving safety in general, there is nothing under there worth dying for.