Trying to reconstruct what happened, investigators said the bus was apparently in the right hand lane and the truck was in the center lane, passing the bus. At the rear of the tractor-trailer was a step, on the right hand side, and the driver said he believed the step clipped his front bumper. Investigators said, however, that in swerving to avoid the back step the driver might have inadvertently initiated the accident, swerving, then striking a guard rail, flipping over and slamming into the stanchion.