As peachy said,

Nightmares can be caused by acute stress. That stress can be lack of oxygen to the brain. This is where knowledge about your breathing while sleeping is important. For me, trying different sleeping positions made a big difference.

I do best flat on my back with minimal head elevation and my pillow stuffed/clumped against my right cheek so my head can turn slightly to the right to rest on that pillow clump.

I also do good in a recliner.

So, there may be more than one reason for your nightmares, both physiological and psychological.