What actually pushes your bus down the road?--think about it for a minute.
This is gonna get a little techie-but bear with me...
My (great) shop teacher at tech school was old--and always seemed to have a story from the old times to give as examples. He was telling tales of when cars cars had torque tube rear ends. (The tranny is solidly attached to the rear via a large tube with the driveshaft inside it.) What would happen is if the motor mounts failed, the motor would get pushed forward (because its attached to the tranny which is pushed by the torque tube which it pushed by the rear axle) and jam the fan into the radiator. The motor mounts were the part that actually pushed the car down the road.
My race car has a three link suspension. (one bar on each side below axle centerline and one in the middle up top.) I have a 1200 lb spring in the upper link. It gives a bit of cushion when I launch.When the pinion gear turns, it tries to climb the ring gear-and the rear axle rotates backwards. Where the lower links attaches to the frame-is what pushes the car forward.
How much force is on the parts? All of the engines torque-multiplied by the ratio in the tranny + rear. On my 1200 lb car-with 100 ft lbs of torque I move that spring an inch.
Now picture the amount of force 500 ft lbs (multiplied) has on a 30k lbs. Stand at the side of the bus-and imagine the rear axle wanting to rotate backwards-whats holding it back? Just the U bolts. Maybe 1/2" above axle centerline to 5" below at the bottom of the spring perch. That little area is whats pushing your bus down the road.
Since you can still see the the leaf springs in the pic of the bus on the wrecker, that means the u bolts must have failed to hold the rear from turning over backwards. And one pic shows it a few feet past a stop sign, it makes sense it happen at take off-where you have the most load on the parts.