Apparently it's something that can happen spontaneously in tall, thin people. Given that the only other causes they told me about were smoking and a strike to the chest, I'm forced to think that it was a spontaneous one. That doesn't do much for my peace of mind though.
What you're describing is called pneumothorax, and happens when the sac in which your lungs are, which is normally at negative pressure... well, stops being so - so it no longer pulls the actual lung outwards and that lung collapses.
The most obvious way it can happen is if the sac is pierced through a bullet or something - because your lung nor your body is kept depressurized, for obvious reasons. It can in fact happen spontaneously, for poorly understood reasons, and does in fact happen more often to tall males (possibly thin, too, but tall and male are more important) but also literally anyone else - the actual reason is that some weird lung abnormalities form and rupture, and for purely mechanical reasons, it happens easier in tall males.
There's actually a ton of other possibilities - infectious diseases, genetic diseases (including Marfan syndrome, which is interesting insofar as it's a connective tissue - AKA the thing the sac's made of - disorder that also happens to make people tall and Thin Man-y, so there might be a link too, I suppose), all kinds of stuff.
That's actually the terrifying thing here. We are squishy.
Squishy as fuck.