I've driven that bridge many times since I live in South County. Where the highway connects with the bridge or just before it there is a jolt to the vehicle you are driving. Also, if at that particular time you are also passing a car in the slow lane, the bridge seems very narrow at that same spot. It could be an illusion because of the curve in the road.
When I am driving my husbands 04 Chev 4x4 instead of the midsize SUV I am use to, I can get nervous just approaching that spot.
Possibly the driver was caught off-guard by the jolt and lost control of the bus. I can see how it would happen.
It would be interesting to see if any of the investigative officers know the area I am talking about.