Inexperience of drivers tends to be a large one. When I say drivers, I don't mean the bus drivers, I mean other drivers on the road and their inattentiveness. A lot of the collisions happen at intersections. Personally, I've never investigated a bus-related collision that did not happen at an intersection, so people not paying attention and driving through a stop sign or whatnot is typically what the root cause is.
On April 2nd, 2019. See this statement in context.