It sounds like the car attempted a lane change, but cancelled when it detected the bus. The bus didn't want to wait for it to return to its lane, so it just scraped the side of the car. If the car was further in the lane, it would have been rear-ended, but the report just says side scraped.
Note that neither the AV nor the bus ever left the lane. This all happened in a single 'wide' lane, wide enough for both vehicles to be in it at the same time.