Cabin crew only initiate an evacuation in 'clearly catastrophic circumstances' otherwise wait for evacuation to be initiated from flight deck.
Of course there is always the chance that it is not catastrophic but cabin crew are trained to make a good decision and what constitutes circumstances of evacuation.
One earlier comment here refers to undo straps I don't think that is the right terminology that passengers would understand. It should be unfasten seat belts in all airlines.