It seems a common thing in Europe that when braking unexpectedly (even if not an 'emergency' brake) that they put their hazards on.
For example, joining the back of a traffic jam or congestion on a motorway - they will put their hazards on to indicate they are stopping where you wouldn't normally expect them to stop. I though it was quite a good custom. Another good thing they do is when the motorways are congested, the cars in the 'fast' lane pull over as far as they can to the left (would be the right here) and the cars in the lane next to them move as far as is safe to their right (would be left here) so that if needed, emergency service vehicles can get down in between. I can only imagine if we did that here, some entitled plonker would drive straight up the middle :/
I do the hazard lights when stopping at the head of unexpected queues. Some vehicles also activate the hazards under emergency breaking as a matter of course.