The reason of Bitcoin's rapid growth was because of the demands that it got after it's creation.
There were campanies, one by one, that accepts payments, donations, and other transactions.
One of the main reason of such boost on demand for bitcoin was the launch of Bitcoin Foundation.
It was launched to "accelerate the global growth of bitcoin though standardization, protection, and promotion of the open source protocol".
Bitcoin became a trusted currency system by a lot of people that is why the demand growth was high. Resulting for the bitcoin's value to go high also.