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.
The first two will never work on the price of bitcoin if we the users of bitcoin try to understand that bitcoin is a decentralized currency and no one can have a control on it even the governments cannot control it. Their statement cannot bring the price of bitcoin down unless we sell our coin. When they announce any regulation and we do not sell our coins then the price cannot drop and they cannot rule over bitcoin.