This is often due to a sugar that cruciferous vegetables contain called raffinose, which produces extra gas as it breaks down and can make you feel bloated. (goodwitchkitchen.net)
Prebiotics, which include other complex carbohydrates such as raffinose can cause flatulence. (sugar-and-sweetener-guide.com)
While most humans on earth can ferment low levels of raffinose without problems, some are sensitive even to tiny amounts. (supernaturalacnetreatment.com)