Fox News had a report on this last night. It seems that when casava is prepared, it's possible to do a bad job of cleaning it and leave cyanide residue, cyanide being a naturally occurring part of some of the casava plant.
Fox news got a little bit confused: The plant contains cyanogenic glucoside, which releases cyanide when "disturbed". It takes 150° or 3 days (I believe) to complete the reaction that releases the cyanide.