Healthy gut flora create a competitive environment in the gut. They excrete various acids and other substances that make it harder for pathogens to attach to the gut wall, and ones that can prevent them from creating biofilms or outright kill them. These substances also lower the pH in the intestines to a level that isn’t friendly to many pathogens, including fungal candida. In addition, they help regulate immune responses and regular bowel movements, and keep the gut and the rest of the body healthy in a variety of other ways. This is why dysbiosis is being linked to everything from obesity and diabetes to arthritis and cancer.
Candida isn’t particularly competitive, so in this kind of environment it’s completely benign. It’s not waiting for anything, it’s just munching on leftovers as a harmless yeast. If you take away the competition, raise the pH, disrupt immunity and provide plenty of food, then it’s able to transform to it’s fungal form with a free pass to grow in a way it wouldn’t usually attempt or be allowed to. Antibiotics can bring this effect about pretty much overnight. This is my basic understanding of it anyway.