1. Government policies and regulation (sometimes banning of bitcoin), like Russia, India, S. Korea and others.
2. News and media sites ( closing of related bitcoin site and hacking like $60m cryptocurrency hacked https://www.wikitribune.com/story/2017/12/11/technology/nicehash-ceo-speaks-out-after-60m-cryptocurrency-hack/27212/
3. Intended market manipulation. Some player buy large amount of bitcoin it causes the bitcoin rise in the market.
4. Limited supply of bitcoin not more than 21M tokens, in high demand.
5 Other policy of central bank.
This is few reason why bitcoin alter rapidly.
I think that's some of the reasons that affect the bitcoin price in addition to the fact that bitcoin is a crypto currency that has a fluctuating price character