The reason which i think is main is its demand in market. And demand is directly dependent on the country policies, if the policies are against crypto then the demand in that country will be very less and the price will get decrease.
Market is Volatile, anything can happen at any given time, market price value of ALT coin can drop or goes up dependent on the Volume of Investment by the investors, the issues on Banned, Hacking, Scammer, and the issues on Regulation by the government critical on the digital currency transaction of Bitcoin in their respective country. all of this indicators affect the market price value of Bitcoin and other crypto.