PTSD is a medical diagnosis, established in 1980, defining symptoms that last at least a month after experiencing a major trauma. These symptoms include remembering or reliving the trauma when you do not choose to; feeling numb and withdrawn; and, having forms of anxiety that interfere with daily life."

Source : Dart Foundation

The mechanisms by which severe trauma can induce PTSD are still unclear.

Increasing levels of stress produce an inverted U-shaped effect on memory retention. Humans experiencing an acute trauma may develop posttraumatic stress disorder (PTSD), but the key contributions of trauma to PTSD formation are still unknown."

Source : Journal of Neuroscience, 35 (48)

Further, some individuals go on to develop PTSD whilst others (exposed to the same stressing situations) do not :

Stress induces a shift from hippocampus-based “cognitive” toward dorsal striatum-based “habitual” learning and memory. This shift is thought to have important implications for stress-related psychopathologies, including post-traumatic stress disorder (PTSD). However, there is large individual variability in the stress-induced bias toward habit memory, and the factors underlying this variability are completely unknown."

Journal of Neuroscience, 37 (8)

Also see : EMDRplugin-autotooltip__plain plugin-autotooltip_bigEMDR

EMDR (Eye Movement Desensitization and Reprocessing) is a form of psychotherapy, which uses types of bilateral sensory input - typically side-to-side eye movements. It is intended to diminish negative feelings associated with memories of traumatic events, and is frequently used in cases of PTSD.