If you make the up tube large so that the speed is arbitrarily slow then you won t have flying droplets rather the water would just dribble out of the end of the pipe With a garden sprinkler and jet as water exits the sprinkler jet pressure energy is convert to velocity head In the siphon if your pipe is large and there is no jet then there will be no flying droplets You need either the water to be travelling at a reasonably speed if there is no jet or you can have a large tube and slow speed if you fit a jet however as noted above pressure energy is converted to velocity head energy as it travels through the jet with the smaller jet size giving it the speed necessary So the answer to what makes the droplets fly through the air is velocity head energy