Apparently, taller men are more likely to sire sons than daughters, and are also more likely, historically, to survive a major conflict. Therefore, the survivors are more likely to give birth to sons, leading to this effect.
(Also, it seems that as modern warfare has changed in that being taller is no longer an advantage over being short, this effect has disappeared.)
Alt speculation: stress alters female hormone balance during gestation and favors an androgenic environment, which would lead to a higher incidence of “male” XX development. Most people are never karyotyped and wouldn’t know if they were XX anyway.
If either of those explanations were correct, we would still be seeing this in families where the dad was returning from Iraq or Afghanistan. However, we aren't, suggesting that the real cause lies elsewhere.
You'd think being surrounded by men so much for so long their genes would be going "there's a clear lack of vaginas in the world at the moment" but apparently not
If so, I can think of a few potential reasons
(Also, it seems that as modern warfare has changed in that being taller is no longer an advantage over being short, this effect has disappeared.)
"the gender ratio remains high even a few years after the end of the war."