Utilisation - EK384 arrives in BKK at 1215. If it went back to DXB at say, 1400, it would only arrive into DXB at 1700 local time with nothing to connect to.

By departing at 0115, arriving at DXB at 0445, it can be prepped for immediate departure in the morning bank - layover times are also shortened for passenger demand as well.

That leaves a ~9hour gap where it could do nothing except cost money on the ground in BKK. So, you look for a 3hourish sector ex-BKK where you might get good pax demand. HKG is the obvious choice.