It's common knowledge amongst drain cleaning plumbers that a sudden infestation of baby fruit flies in a house is generally a sign that there's a major leak in a drain pipe (most commonly the kitchen sink drain pipe).
You see, the reason they're called "Fruit Flies" is because they lay their eggs in moist foods of various sorts, but not necessarily fruits. Lots of foods go down the kitchen sink drain, and if they're accumulating in a pile in a crawl space under the house, that's the perfect pile/place for a fruit fly to lay her eggs.
Any time you suddenly get an infestation of small little flies (baby fruit flies) in a house, the first thought should be: "When was the last time I tossed out the garbage?", and "Could the drain line from my kitchen sink be dumping all the stuff that goes down it into a pile in my crawl space, and is that pile now incubating and nourishing a new generation of fruit fly larvae (commonly called "maggots")?"