Why would volcano eruption cause cardiac arrest?
Michael's comment is correct, in Japan, according to the article Japan volcanic eruption: Mt Ontake rescuers find more than 30 people in 'cardiac arrest' (Radio Australia), state


  Police said the 31 people were found in "cardiopulmonary" arrest on Sunday but declined to confirm their deaths pending a formal examination, as per Japanese custom.


and more directly, from More than 30 feared dead on Japan's Mount Ontake after volcano erupts (Washington Post)


  using the terms for heart and lung failure that Japanese authorities employ to describe bodies. People cannot be declared dead in Japan until doctors have examined them.


It is very likely, given how close they were to the eruptive crater and the sudden inhalation of hot volcanic ash, gases and other materials, the victims went into cardiopulmonary arrest due to their lungs being overwhelmed by the sudden quantity.  Additionally, burns, fear, debris could have likely caused shock causing cardiac arrest.

Several case studies from other volcanoes are presented in the article The respiratory health hazards of volcanic ash:
a review for volcanic risk mitigation (Horwell and Baxter, 2005).