This doesn't strike me as being such a mystery.
As a generality, the vast bulk of the Army would be comprised of men. Thus leading to another generality that by and large men will be the ones getting hurt in theatre. Also a generality, if a man is married, chances are he is married to a woman.
Now, with those basics (and this is in no way an invitation to debate any aspect of the a/m paragraph) ideas, by and large you have men going off to potentially get hurt.
Regardless of gender, the ones who stay back have a hard time with it. If you are deployed, you get to focus on your job, and do what you need to. The loved one back home has no control over what is going on, and is way out of the info loop. They just have to wait from phone call to phone call just to hear your voice and know you are alright one more day.
We also have to assume that everyone who has a loved one on deployment inevitably engages in running worst case scenarios through their heads. That knock at the door, and seeing people in DEU's and knowing bloody well why they are there. You can create the nightmare ahead of time, then end up living it without the luxury of waking yourself up. So in that, the stress portion is there for the spouse.
Next, couple that with the fact that it is always so much harder to see someone you care about in pain than when you are in pain yourself. I would have my leg broken a dozen times again if I could never have to see my 2 1/2 year old daughter screaming at me, pleading with me as I pinned her arms while a nurse had to put in the tinyest and most horrible IV I have ever seen into the back of her little hand. Tears running down from her beautiful blue eyes that are looking square into mine with hurt and betrayal saying "No daddy, please, I'll be a good girl, please don't let her hurt me, I just want to go to bed, please daddy, it hurts". Fighting back tears because I didn't want her to be more scared, then fighting back a murderous impulse to smash the nurse into mashed potatoes when she couldn't find a vein six times in a row as I realize that they put a student in the breach to "practice" on my girl. That shit haunts me to this day, and always will.
So for a spouse, having to sit by and watch all manner of Hell being repaired on a loved one (possibly a son or a daughter) it is the helpless, gutwrenching pain that you can't do anything with. I don't think for a second that couldn't lead to PTSD in any manner of forms. Depending on how well an individual deals with stress, just hearing reports on the news of incidents could be driving huge nails of angst into an individual even if their loved one wasn't involved. I know if I end up over there, there will be people who will be afraid for me from the time I lift off to the time I am back on Canadian soil regardless of what I tell them is going on or how well I think things are. I think it is unreasonable to suggest that just because they aren't the ones engaged in theatre that they don't have any just cause (my words) to suffer from a stress disorder.
Let's not get too hung up on who "gets" to be ill or not, and just work on taking care of whomever needs it.