Dave Petley alerted readers of his landslide blog to the British Geologic Survey initial assessment of the coal pile failure in South Yorkshire http://www.bgs.ac.uk/science/landUseAndDevelopment/landslides/hatfieldFeb2013.html.
I had enjoyed my own long distance assessment of the failure coal-trains-landslides-and-long-distance-geology. The BGS site followed a similar approach which was fun to see, but I included the soil map of the area in my write up. The BGS did go through some old maps of land use in the area and noted the presence of brick kilns and significant drainage on farm fields. Both indicative of soft clays underlying the site. So the main suspect is the underlying soft alluvial soils failing under the coal pile.