Anyone can get a low-level yeast infection but yes you are right, there must be an underlying cause for it to get this out of hand. In the case of the Mirena,the hormones are the underlying cause. It's a constant supply of synthetic progesterone and it's well-known that hormones feed candida. Even ladies who don't use hormones who suffer from candida are worst just before they come on their periods (i.e when progesterone is at it's highest.)
I probably originally had candida for a number of reasons, too much sugar, smoking, birth control pills. But I didn't get SICK sick until I had the Mirena. Candida isn't an issue until it gets really out of hand.
It's funny how the Mirena only makes certain people sick, and most have no problems with it. I'd be willing to bet that's because not everyone has an underlying candida problem.