First there was a huge increase in users regeristing with My.Gox which started to cause the system to lag, then there was a huge sell order in the range of 5000 bitcoins which caused the price to go down as they were sold and created latency as it filled current buy orders.  That initial large drop greatly affected the price causing nervous traders to sell because of this and the current price at the time.  The continuous sell from nervous sellers, selling lower and lower to get out from under there bitcoinx crashed the market.  Then Mt.Gox shut down trading and has been repeatedly hit with DDoS attacks causing further doubt in the trading system most base bitcoin prices on.