I don‘t think negativity necessarily has something to do with trauma. Negativity bias is very widespread, regardless of previous trauma. Basically everybody flies at negativity.

Bad and shocking headlines click way better than positive ones, negative feedback is occupying our attention more than positive feedback, we perceive losses way more important than gains, we perceive losses as way more impactful than gains of the same degree, etc.

I am 100% sure trauma can and does affect the negativity aspect of our thinking in a big way. But I do not think that negative thinking overtopping positive thinking is limited to trauma sufferers