Passenger on the news said the captain said evacuate evacuate over the pa so I doubt it was precautionary.
What else was the Capt to say. It is either evacuate so that the cabin crew know exactly what to do or come back and have a group discussion over a cup of coffee in the galley.
SOP are there for a reason. Cabin crew yell so as to reinforce what is required.
design overhead lockers that lock ...
Could also help in turbulence