A common question in the field is why some people develop PTSD after being traumatized and why some do not. There’s no shortage few theories that attempt to explain this. Some of these theories sound very sophisticated: they may make reference to elements of biology and neuroscience, making mention of things such as the brain, neurons, and stress hormones.
In all honesty, though, there is no unified theory that comprehensively explains what causes PTSD. All of them, at best, contain fragments of some greater truth.
Consequently, I think it’s simpler to say that: people develop PTSD when they encounter an experience that overwhelms their ability to cope. There’s a number of interacting reasons why this occurs: intensity of the experience, history, coping skills, social support, prior experience, and biology. However, no one reason predominates.
In sum, the most honest answer to what causes PTSD is, “We don’t know.” It’s a phenomenon that is not easy to predict. What’s more important is finding ways to deal with it.