Kinda feels like age of the people may have been a factor here. Segways do require good reaction times and constant awareness of your surroundings because they don't stop (even when stationary they have power to the wheels keeping them balanced, and if you move they move) and it the tour guide thought anyone was too young, old, or otherwise not capable to control the vehicle, they shouldn't have let them go out in public.

Not saying that was the only factor, it could easily have been the tour guide not giving adequate safety instructions or simply people ignoring those instructions... or a mixture of all three!