Either the bike was put on the bus rack incorrectly, or the rack is defective.

I've been using these bus racks for years. I take a bus that regularly gets up to 70mph on a freeway. Never had the bike fall off. I always sit at the front of the bus to keep an eye on it, mostly to keep people from snagging it at stops.

Pay attention to the instructions. There should be a slot for the front wheel and for the rear wheel. Then there's a bar you pull up and over the front wheel. Pull it up all the way to the fork crown. Then give the bike a good jiggle.

They're supposed to be compatible with anything from 16" wheels to 28" wheels.