Thanks once again to Peter Diehl, the causes of failure of the Ajkai Timfoldgyar / Kolontar tailings dam are becoming clearer, and appear to support initial suspicions. There is an excellent gallery of images here, of which this is the most interesting in this context:
This is the damaged (cracked) section of the north side of the dam. Zooming into the cracks on the top of the dam shows this:
The pipeline running across the top of the dam is clear. Note that where it crosses the main (left side) crack the pipeline has been detached from the supports, which have moved forwards and downwards. A little further to the right the pipe has ruptured, and tailings have spilt across the dam surface. Thus, it is clear that the dam has moved forward and subsided at this location, which further supports (though does not confirm) the suggestion that the dam collapse was associated with failure of its foundations.
Interestingly, some suggestions are also emerging that this section of the dam had been leaking ahead of the collapse. WWF has an aerial image taken in June this June of the dam that appears to show a small leak:
However, this should be treated with some caution as there may be other explanations for this apparent spill of tailings on the downstream side of the dam. Finally, there is a gallery of image here that shows the state of the dam in 2005. These two give a pretty good idea of the general state of affairs at the dam: