Mainly it happens with demand of Bitcoin. Bitcoin is increasing day by day rapidly for it has great demand and it is almost stable though the price of bitcoin is not stable. If the supply is less than demand, then Bitcoin will be rising continuously.
It is very simple and basic theory of economics that when supply is more than the demand then prices dumps while whenever the supply is less than the demand then the prices pumps up. In present time we can see that bitcoin popularity is increasing from time to time. and therefore we can expect that bitcoin price wil continue incrasing for a long long time.