premise: More than 50 Polish and Laotian students demonstrated in front of the US embassy here Wednesday to protest the "occupation of Laos by Vietnamese troops," reported the news agency PAP.
  The students also called for free elections to be held in Laos.
  Chanting "Free Laos, down with communists," the Laotian students handed US ambassador Nicholas Rey a petition demanding the United Nations set up a commission to examine human rights violations in their country.
  According to the demonstrators, the United States is the only large country who can oppose civil rights violations in Laos.
  The demonstration ended peaceful without police intervention.
   What caused the demonstration?
hypothesis: occupation of Laos by Vietnamese troops