What are some possible causes to the rise in male births this past quarter century?

More males are born in stressful situation such as war, poverty, etc. Males tend to have a higher mentality and tend to be more oriented towards fighting. Such as war.

Naturally more boys are born than girls. The ratio is 105 males to 100 females.

"Several factors influence whether a sperm containing a Y sex chromosome or one containing an X chromosome will be first to fertilize an egg, including parental ages, their environmental exposure, stress, the stage in the mother's ovulation cycle and even whether she has had children previously"

Countries like India and China, that in someway control the amount of kids people have, are more likely to abort a pregnancy that would result in a baby girl than a baby boy. Leading to an abnormal radio of boys to girls.

Indians and Chinese practice female foeticide. These 2 demographics are there all over the world. That might be the reason.