UA doesn't fly to Istanbul; that's a Turkish Airlines flight. Either there was a schedule change that UA is interpreting as an early departure, or United is reading from a system that's showing an early estimated pushback.

Early departures are quite common -- they're just not usually announced. But if you check the flight status after departure, you may find a 5 PM scheduled departure that pushed back at 4:55 PM because everyone was boarded and the paperwork was ready promptly.