To me it seems as if the traca's ingredients would show some kind of "burning frequency".
My guess is that the firework acted like a flash, with a strong instantaneous light that freeze the people and then while people moved the remnant sparks exposed the sensor but were not bright enough to lighten the surroundings. That's why you have sparks on top of people.
You can see in the first photo some lights that give a hint of a rather long exposure.
What you both say is exactly what I think: the traca is a sequence of fast explosions. The exposure time I recall was 1/8, time enough to include a couple of explosions (if we look closely we'll see some ghosting artifacts in the moving people derived from lightings of different intensity). I see indications why the sparks are not likely flying over the people's head in those pictures (they do in general, and it's not dangerous, but didn't happen in these two cases):
- Most sparks seem to be contained in a ~4m radio sphere, while people are much farer from the centre of explosion than that. Only sparks much faster than the rest could reach the people at that time (this is not impossible, but not likely).
- On the other hand, and specially in the second picture, the spark trajectory is a bit erratic. This indicates a quite low speed which is contrary to the previous idea.
- I don't see a clear perspective shape in the sparks trajectories: sparks coming towards the camera should clearly show wider traces in their end close to the people. In the first image I could have a doubt on this.
- Finally all the "unexpected" sparks fall over people that were moving, which enforces the idea of the
flash effect.
Regards