Passenger Ignorance
As an engineer of 30+ years not only have I worked on aircraft but spent a good proportion of my time flying in them, in fact around the globe several times.
I feel that the staff did not possibly communicate the reason for repositioning passengers within the cabin very well, but then as stated above the captain does not need to justify his descisions to the passengers. he makes hundreds of descisions every flight all as important to flight safety as that one.
There is however there is also a large factor of media hysterics which feeds across to the passengers who believe the media tripe.
You pay to have airlines transport you around the world, they have been doing it for a long time now and it is very closely regulated so place some trust in them.