I stumbled across the tropes page from "A Brother's Price" the other day. Basically, the novel takes place on an alternate Earth in a country called Queensland, where only around one in every ten births is male, all the rest being female. What could cause such an imbalance in the human male/female birth rate?
It can be antibodies in the mother's vagina that target Y-carrying sperm. This is easy to comprehend and there are anecdotal stories of this sort of thing.
It can be down to something similar to Rh factor, if blood types and gender are genetically linked. This is not known to occur, but a little genetic shuffling in a fictional world can make it a realistic outcome. You'd also need to play with family size; Rh effects impact later-born children more than first children.
It can be down to a fictional physiological response in men: Consider, as population declines, men start producing far more X sperm ... because we need more women than men to get the population load back up.
It can be down to when intercourse occurs during the menstrual cycle (X sperm out-survive Y sperm in the woman's reproductive tract, so intercourse days before ovulation is more likely to result in a female baby.)
Here is a science article that looks at gender selection of offspring in mammals.
It can be many other things; these are for starters.
You have two choices
Evolutionary, natural selection favored women who birth females, perhaps a matriarchal society where females are highly valued and a women with that rep has a lot of husbands. Or some other reason leading to the same result.
Environmental, take your pick, temperature, light levels, air pollution, purposely taken drugs... it's a story.
You could merge the two, there is a drug available that will 90% of the time ensure a pregnancy is female. Female offspring are highly prized in society and provide a women with status and power. So women take this drug.
Temperature, because they are a reptile species with temperature-dependent sex determination.
The eggs that are too warm or too cold become males. Eggs in the goldilocks zone become female.
Because the species no longer lays eggs in the ground, inconsistent incubation temperature is no longer an issue and so most children are female.