From the Game's Description:
No one really knows what happened, it all happened so quickly. The driver maybe fell asleep, or a drunk driver t-boned the bus; but it doesn't really matter right now. The bus now lays on its side, parts of it engulfed in flame. Glass is everywhere... fuel is pouring out of the gas tank. And some people have been thrown from the bus.
However, I wouldn't let the description above fool you. While a majority of the action takes place surrounding the rescue efforts of a bus accident, "Bus" is an attempt to be more about the consequences of attempting to be a hero, who the people were before the accident, and what they wound up becoming afterwards.