https://bitcointalksearch.org/topic/pboc-reaffirms-banks-to-quit-btc-activity-drop-424-583633
Or as someone said in the thread:
Quote
The real reason for the recent drop was the confirmation of Mt Gox liquidation by the Japanese courts and the possibility of an auction of the remaining Gox coins.
Honestly though do you care? If so, why not buy some insurance or hedge your investment/bet?
These drops happen daily with bitcoin. If we have a massive drop or or a continued run-down, then I'd start asking questions, even then though - are you planning to cash out? What is your target? What is your stop-loss?
Hope that helps.