premise: Seven people were killed in a colliery accident in Southwest China on Monday, bringing the death toll in coal mine tragedies to 85 over the past three days.
  Eleven workers who were sorting impurities from a coal heap were buried when the heap collapsed at 9:47 a.m. at the Shuicheng Coal Mine Group colliery, in mountainous Guizhou province.
  Seven workers died and four were rescued, said the work safety administration in Liupanshui city, where the group is located.
  The cause of the accident is being investigated.
  The past weekend has been tragic with three mine disasters leaving at least 78 people dead.
   What caused seven people to be killed?
hypothesis: a colliery accident in Southwest China