Airbags are not meant to deploy under 11 mph so it's possible you were going less than that. In addition, since the car was able to know that your belt was buckled the computer probably didn't think that there was a need for the bag to go off as the seatbelt had locked up and done its job. The airbag may have gone off on the other side figuring that there was a passenger in that seat that wasn't buckled.