Have you driven an 18 wheeler before, or even learned about them? They don't stop on a dime, they take time to do maneuvers that cars can do near instantly. So if a truck has to slam on the brakes / swerve, there is a high chance of toppling due to the massive load they carry. From the news report, the car cut him off so he had to brake and swerve, and that was too much for the 18 wheeler.\
btw, i was refering to the driver of the toppled 18 wheeler, not the driver of the car that caused the accident.