This is the way I explain trauma to the people I work with.
When something terrible and/or unexpected happens a part of the mind takes on the job of figuring out exactly what happen, why it happened and (crucially) what our role was in the event. This isn't to punish or torment us but to ensure that that particular thing never happens again – it's very much a protective mechanism.
That's fine if we were texting when the car accident happened or the other person was drunk, but some things are just so random or unexpected that there's absolutely no way of predicting them, preparing for them or avoiding them. If there were other people involved who can't or won't explained their role/actions then the mind can only work with it's own interpretation – so at best 50% of the remembered reality.
However that bit of your subconscious doesn't understand that there might never be a resolution and works on and on, rerunning the event(s) from every possible angle – hence the flashbacks, intrusive thoughts, nightmares that are part of trauma.
Once that part of your mind understands that there isn't, and never will be, a satisfactory answer it will stop, because it's a fruitless exercise which is detracting from your happiness. You won't forget what happened but the emotional link will be broken.