It is not just the Hanoi flight. Every single flight in the 1am-3am departure bank has the potential of being delayed overnight because of lack of available aircraft. As overnight arrivals come in, these aircraft are then immediately turned around to operate the delayed flights. The greater the ground time of the aircraft at the outstation, the greater the chance that flight will be delayed because some if not all of the delay will be made up on the return portion of that flight. The only flights that seem to be somewhat (though not entirely) immune of these lengthy delays are the TK198 to Tokyo Haneda (tighter slot restrictions and shorter ground times there) and the TK181/182/184 departures to Mexico (no extra ground time at Mexico and Cancun). Lately even flights with very little potential to make up time such as TK066 to Bali and TK084 to Manila have been victim of lengthy delays.

These overnight delays are an inherent risk of booking TK these days and the only recourse available is the EC261/UK261 for those who started their journeys in the EU or UK.