The passenger reportedly lost his balance. It was not stated if there were any factors leading to the CTA rider falling, such as bus overcrowding or erratic driving, which in my experience as a Chicago injury lawyer can be common causes of passengers losing their balance.
After the passenger fell on the driver, the bus swerved and hit a viaduct in the area of East 130th Street and S. Indiana Ave. The Incident occurred around 9:30 PM. All 11 of the injured were taken to local hospitals with no serious injuries reported.
Read the story as reported by WGN-TV here.