SEOUL, South Korea — South Korean police investigated on Monday what caused a crowd surge that killed more than 150 people including 26 foreigners during Halloween festivities in Seoul in the country’s worst disaster in years, as President Yoon Suk Yeol and tens of thousands of others paid respects to the dead at special mourning sites. Saturday’s disaster was concentrated in a sloped, narrow alley in Seoul’s Itaewon neighborhood, a popular nightlife district, with witnesses and survivors recalling a “hell-like” chaos with people falling on each other like dominoes. They said the entire Itaewon area was jammed with slow-moving vehicles...