The reason for that is the death of Bitcoinica. Bitcoinica offered shorting, which stabilized prices at around $5. When Bitcoinica went out of business, the only way to make money on Bitcoin is to speculate its' price increase which will drive demand; which will drive its' price even more. That being said, it might be the right time to buy Bitcoins, and sell before the bubble bursts. However, for a more stable Bitcoi…
Leveraged trading is trading with partially borrowed money. If you buy something with leverage and the price falls, you're forced to sell. A wealthy trader can temporarily lower the market price by dumping a bunch of coins on the market at once, knowing that the price will recover when he buys them back; this costs him money, because he gets a lower price for his coins and buys them back at a higher price. But it also forces margin traders to sell out their leveraged positions, at the lower price, which those same traders can capitalize on.
These sorts of shenanigans depend on there being unsophisticated traders using leverage (a bad idea for any non-expert trader!), and they get harder as the number of traders in the market increases. Many of the rapid price oscillations we saw were a side effect of this strategy. (Not all of them, of course; the initial rise was caused by the wave of publicity, the fall by loss of confidence when some people lost their coins by storing them on insecure computers or giving them to scammers pretending to be "online wallet services".)