I wonder if the specific tools of war are a factor as well — the suddenness of explosions and bullets might provide a significantly more traumatic experience than spear sword and trebuchet, the pace of humvees and airplanes more shock than feet and horses.
And medieval combat was quite a gruesome affair, cleaving people into peaces with axes and swords, in an age without proper medical care...
What has changed though, is the staying power of armies which nowadays can stay in contact for long times with somewhat low intensity fighting going on all the time. This of course changes the whole psychology aspect, its not the stress of one battle going on for a day or so, but the stress of a battle going on for weeks.
"Gun cleaning" accidents, as the socially accepted cover for male suicide by people everyone wants to ignore were suffering from depression or PTSD.
There's no doubt that happens, but on the other hand gun cleaning injuries (e.g. to limbs) are far from unheard of, as well as people accidentally putting holes in their house. Those are doubtlessly accidents, so I have no doubt that some of the "gun cleaning fatal accidents" truly were accidents. Any of these accidents should be prevented by dogmatically following basic safety procedures, but obviously sometimes some people don't. And sometimes, that has fatal consequences.
All quiet on the western front describes this phenomenon quite well. The continues stress of being on or near the front lines with constant shelling all around you. The artillary barages where so severe that areas of the ground are poisoned with metals from artillery shells..
To give you an idea about the frequency of shells getting fired.[1]
[1] https://codatocoda.com/blog/making-a-new-world-armistice-sou...
Casualties during the battle proper would be surprisingly low.