The quotes below are from what I would call "pop medicine" websites, which I don't think much of in general, but if correct, they shed some light on your question:
"Some medical studies have shown that hormone balance could affect intestinal bacteria makeup. Supplemental estrogen in the synthetic form has been found to promote the growth of yeast, creating yeast infection symptoms."
"Birth control pills decrease the acidity of the vagina and cause the vaginal cells to store excess sugar, both of which help yeast to multiply."