Latefinals,
I know the capt on that flight and I spoke to him a few days after this mentioned flight.
First they had an aircraft change due tech problems before flying to Turkey which ended up in them having a plane with a lower max takeoff weight (64 tonnes) instead of one on that kind of route distance which is normally flown by a 68 tonne aircraft.
For that reason they were restricted on their takeoff weight and had to decide whether to offload passengers or refuel en-route. They obviously chose the last option which I'm sure all the pax would appreciate! Yes they also had a strong headwind too.
Hope that explains you why!?
PS. Didn't know EY (Etihad) flies Istanbul to Luton
