Demand and supply, as with any other good/stock/currency.

However, BTC is a subject to a lot of manipulation, speculation and rumors that can easily destabilize the market. That's where you get such huge volatility.

BTC is still immature, it's influenced by emotions, hopes and even greed that affect demand&supply levels. No other world currency nor good can go from $60->$1100->$400 in such a short period :-)

Until you clear your browser cache... :-)