What is (C-)PTSD? (Complex-)Post-Traumatic-Stress-Disorder

(C-)PTSD (from now on only referred to as PTSD) is a reaction to a trauma. That is it. You don’t need to be a soldier or have a horrific things happen to you. Usually the intensity of the symptoms parallels the trauma.
Usually a trauma is an experience that your brain doesn’t know where to put it. It can’t basically stomach it. It might be because of progresses that happened or it never have encountered anything like it before.
Especially things in the childhood can cause such a reaction, as the child mind is easily overwhelmed and is only learning what is normal and what is not. Things not a problem for an adult, can become a trauma for a child.
The not knowing what normal is, is the reason are abused children rarely aware, that that was is happening to them, is not normal and shouldn’t be happening.
It also might lead to them being completely accustomed to the PTSD symptoms and not being aware that those symptoms are a red glad/warning sign.

When does and doesn't a Trauma become (C-)PTSD?

From Episode 95:


Before we get into it, let us preface, that of course it is not meant as a fool proof method. A trauma is a very individual and personal experience, so there is always wiggle room that might tilt things or you might not be aware of all the factors.
This is supposed to be a recipe of factors that cause it and with many factors there is just always a room for error. I will introduce the factors that play a role in this and how they are affected.
This is not what kind of trauma it is, that is what we talked about last episode, just explains how big it becomes. As in theory, every trauma basically causes PTSD sort of, but usually the mind has mechanism to resolve it on its own.
Which is why we get only a PTSD diagnosis after the symptoms last several weeks. You basically only get PTSD - by the official standards - if the trauma is too big for your own system. Which is when additional steps are necessary.
And this leads us to the factors that determine the size of the trauma, first we get into the factors of the person affected and then the factors of the trauma itself.

Factors of the person
As mentioned we start with the factors that affect the person itself.
1. Brain capacity: Which I would argue is the most important factor, as it the backbone of it all. This factor is lowered by stress, tiredness, troubles, hunger and the like. This factor also determines the speed of your recovery and your ability to tackle PTSD.
2. Preconditions: This factor is lowered by having preconditions, be it other trauma before or any other trouble you keep under wraps. Suppressing something drains you and makes you more vulnerable to future trauma effectiveness.
3. Protection: How good was the brain able to shield itself? Did it had earlier experiences how to deal with a situation like this? This factor is lowered, when the brain is not able to shield or prepare itself or the like regarding the trauma.
4. Stability: How stable was the soul construct before trauma hit? Basically, how secure was your character when you got hit by it. People in change or in crisis are more vulnerable. So this factor is lowered by a lack of stability.

Factors of the trauma
And Second we get into the factors of the trauma
1. Length: The longer, the less energy reserves are available and the less the brain is able to protect itself. Over a very long period of time it becomes basically impossible not to get PTSD and even get CPTSD in most cases. So this factor is lowered the longer it takes.
2. Severity: How severe was the trauma? A pretty straight forward factor, which is lowered the more severe a trauma is.
3. Complexity: Where there several factors involved? Be it other traumatic experiences or just things that take a situation to a worse level. This factor is lowered the more complex a situation is.
4. Aftermath: Was the brain afterwards able to relocate resources? And calm down? Giving the brain some happy impulses and free space after a trauma can help tremendously - as studies have shown. This factor is lowered the more stressful the aftermath is.

So to recap: There are several factors that determine the size of the trauma. The lower the factors are, the bigger the trauma and they do add up easily. As much as many factors are out of our control, having a balanced mind helps you the most.
The smaller the easier the recovery and possibility of healing and curing. The size can vary greatly with so many factors, as you can imagine, but even a remotely sized trauma will cause PTSD, because of the brain capacity factor.
Brain capacity is relevant twice, as it determines how much a trauma affects us - aka its size - and also if the resulting trauma is too big and therefor causes PTSD. The more brain capacity available - the bigger the trauma that an be processed.
That has of course limitation and if it takes too long, no brain capacity will be large enough to deal with it. Nonetheless I hope this answer your question why some trauma turn for some into PTSD.