Waterloo Regional Police responded to the area of Arthur Street North, just north of Waterloo, around 1 p.m. Monday.
Police say a transport truck was travelling northbound when it stopped for a school bus, and the SUV driving behind the truck did the same.
The second transport truck behind the SUV allegedly failed to slow down, and struck the SUV, pushing the vehicle off the roadway and into the ditch.
Both people in the SUV were transported to hospital - one sustained serious but non-life-threatening injuries, the other was treated for minor injuries.
The investigation is ongoing, and charges are pending.