@Silverchild79,
The bus probably didn't tip further over the edge because it was wedged against the gaurd rail ( described in the story) and was positioned behind the obvious break in the pavement. The tractor was just over the break and tipped, but the trailer was not. IF the tractor-trailer rig was carrying a load, it should have had equal weight over each axle.
They would both FALL at the same rate in this instance because they were falling with the roadway, not freefalling down to the water.
Lucky happenstance that the kids were not injured, of course, but a miracle? Not IMHO.