Not that long ago, I was talking with an airline passenger who was complaining about a major US airline carrier having "a habit of" moving flights earlier (that is, the opposite of a delay) to the point where he (and his party) were missing flights, including one connecting flight that had been moved up so early that it boarded while he was still on the incoming flight which was according to its original schedule. I think he was talking about all flights within the US.
Subsequently, I showed up at a US airport 4.5 hours early for a flight figuring that would be plenty of time, but after arrival found that the flight had also been moved earlier by more than four hours, and the gate had changed from one end of the airport to the opposite end (as far apart as two gates could be with an indoor connection). So sorry! Also, it's the last flight of the day on that route.
Under what conditions do airlines do this? It sure would be helpful to be able to better predict such occurrences in the future, or at least understand the causes.
Edit: This question was flagged as a duplicate of that one, which focuses on airlines moving up flights by five or ten minutes. The present question focuses on airlines moving up flights by e.g. an hour or few.