Since I started with bitcoin saw the rapid rise and decrease of the price, so as I read the news and other blogs about bitcoin this is the things that constantly alter bitcoin value.
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.
Fake news and FUD's. Many fake news about bitcoin been shared through the web and nothing can stop this because it already started and so as the people who interfere with bitcoin's progress are multiplying exponentially. All the false accusations about bitcoin was not comprehended by some countries so they choose to either ban it or regulate.