This link (MediRabbit) says this: "Fur mites appear associated with spring when weather becomes milder, with a lack of vitamin C due to e.g., stress, or due to an underlying disease, which suppresses the immune system."
What kind of mites are your rabbits getting? If it's Demodex mites (which tend to be more common) they naturally live on the rabbit and only overpopulate to cause problems when the rabbit's immune system is compromised or suppressed. Demodex is common in very young animals that haven't yet built up a strong immune system for this reason. In dogs, Demodex is typically treated with medicated baths, but I am not sure how comfortable most vets would be using this technique on a bunny. Sarcoptic mites do not live naturally on the rabbit and would recur because the medication is not strong enough to break the life cycle or because the rabbit is being continually reinfected by their bedding, toy, or pretty much anything they came into contact with while having the mites. Sarcoptic mites are pretty rare in pet rabbits, however, and I have no idea how she could have gotten them unless she picked them up off your clothes from a wild rabbit or infected pet, or if she's new to your home and picked them up in a previous location. Good luck!
I think the question is what the vector is, they don't pop up out of thin air. I don't know for sure, but I suspect that rodents like mice can spread them, didn't read up on that. But I don't have another explanation for how my buck got mites, there are no wild rabbits for miles, so something else was responsible.
If there are no wild rabbits around, it's probably Demodex which is a natural fauna living on rabbits. A vet can medicate to reduce the population and build up the immune system to prevent another mite overpopulation! They also can't get other kinds of mites from mice because each mite species evolved to be unique to a certain mammal. So rabbits can't get a full-blown Sarcoptic mite infestation from mice, dogs, cats, people, and they cant spread it the other direction either, just rabbit to rabbit.