Continuity mistake: When Harry and Lloyd fall through the ceiling, they start to walk down the hall, away from the ceiling debris, for about 20 seconds, but when the camera angle changes to their backs, they're next to the debris again.
Other mistake: When Lloyd asks Harry about the geeky girl, the clip of the girl is in reverse, hair shows the effect. (00:08:55)
Answer: Because Harry and Lloyd had poured gasoline all over the car as well as let the pump keep running when they left. So when the hot ash of the store clerk's cigarette hit the gas fumes, it caused the explosion.
Bishop73