No survivors in Quebec slide – The Globe and Mail.

The ancient ice age has taken its modern victims. Only the family’s golden retriever, tied outside to a tree, survived.

The article linked above describes the land left behind when  the inland Champlain Sea receded,  and how the clay shifted, creating the sinkhole. The clay itself is  well described in a Wikipedia  article. The link follows.

http://en.wikipedia.org/wiki/Quick_clay

The Champlain Sea extended to the Upper Ottawa Valley. A map is available here:

http://hoopermuseum.earthsci.carleton.ca/2001_champlain2_mb/history2.htm

Minor earth tremors, human drilling, and excessive rain and snow, are among the factors that can change the quality of the clay in an instant. I grew up in the Ottawa Valley, not knowing the land below could be so treacherous.