There is a lack of liquidity. As the price moved above the level where it was profitable to mine bitcoins, prospectors moved from creating new bitcoins to buying existing ones. This simultaneously push up the price and reduced the overall liquidity. The effect was an acceleration of the price once this hardware mining threshold was breached. So in summary, the appreciation of bitcoin can be attributable to the interaction of these three components. Add to this straight speculative frenzy, visible in all bubbles, and you have the history of bitcoin so far.