The mine produced pellets. The waste is silica (sand) so toxicity is not an issue.
The disaster happened at dusk, all they would have know until the next morning was that there was a major tailings dam collapse.
As to cause, the obvious ones are earthquake, abnormally heavy rain and structural failure. Brazil has few earthquakes and recent ones are far away in Chile. Could not find anything about recent heavy rainfall. The dam was inspected by regional environmental office in July 2015. Samarco has its own tailing monitoring crew (they would be in the hot seat at the moment). Management would currently be focusing on disaster relief and clean up. It will be some time before an explanation is identified.
Not keen on 50:50 JVs - who takes responsibility. To paraphrase a Russian proverb - "owned by everyone, responsibility of no one".
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