There is no pre-programmed speed that will set off airbags. It's not that simple.Because it was not a hard enough hit and the sensors did get set off. Probably just to each side of the biggest indentation. Notice the side of the bumper hardly moved, at all. Guessing, at moment of impact, speed was 15mph or lower, or the sensors themselves detected it as such.
Anything over 15mph, they go off or are supposed to, last I knew.
Called the 5/15 mph Bumper, back in the day. speeds of 5mph or less,no damage should be detected on vehcile, over 15mph, bags deploy and fuel shuts off.