Passage: On 25 June 2017, a tanker truck exploded near Ahmedpur East in Pakistan's Bahawalpur District, killing 219 people and injuring at least 34 others. The truck overturned when its driver attempted to make a sharp turn on the N-5 National Highway. Once the news of the accident spread to nearby villages, hundreds of residents rushed to the scene to loot the truck of its cargo. The truck then exploded; early reports suggested the explosion was caused by someone lighting a cigarette.

Question: What caused the explosion?
someone lighting a cigarette