Usually it comes down to too much leverage, a bit of bad luck and too many collrelated positions that cascade. One of my best clients lost 150mill one year but was very close to making that much as he was close to getting a couple of his targets hit (I think 1 or 2 pips in a Euro/kiwi for one) the positions he had turned around very quickly and left him fucked. As this guy was a star trader and had been for many years he was given certain risk dispensations that came back to bite the hedge fund in the arse.