It's not so much that military spouses are predisposed to being terrible husbands/wives, it's the circumstances they often find themselves in that provide challenges that are less common in the civilian world.
New military spouses are often young, with both the service member and the spouse commonly being substantially younger than the national average. With this youth comes a lack of emotional maturity.
Military families are faced with extended separations during times early in marriage within which it is the most important to be together, figuring it out. Relationships are often still developing, and a solid foundation doesn't yet exist, when 20 year olds are separated for 6 months to a year (and sometimes longer).
During this period of separation, new/young spouses are often alone, and lonely, in the middle of a highly sexually charged environment full of aggressive, alpha peers for very long periods of time.
Put this all together and I think anyone can understand why so many military marriages self-destruct early on: You're 19 years old. You just got married to someone you've only known for six months, mostly out of a sense of urgency due to an impending deployment. You don't really know this person very well, and all of a sudden they're gone... for a year... You hold up well for the first couple months, but now you're bored, very lonely, and you are literally surrounded by thousands of men/women, most of whom are unmarried, many of whom are available, and some of whom show their interest. Perhaps daily. Without a solid relationship, without the maturity and grounding that comes with surviving your teens and 20s, this is a disaster in the making.
In the civilian world you still have temptation, and you still have relationship challenges, yes, but they more come individually, not all at once, for extended periods, during seemingly endless separation.
Older military spouses are more insulated against this whole thing.
Source: Was army, deployed, saw it happen to a lot of families. Noticed younger spouses commonly self-destructed, older spouses generally persevered.