Because the cryptocurrencies are ruled by the Demand and Supply law , higher demand tends to increase in price and decreases when supply is not enough. The market of crypto is like a glass full of water that tends to overflow when added (mined) , now every movement in cryptocurrency can affects to its value.
Every market is ruled by demand and supply,be it gold silver or any commodity market ,the demand and supply determines the price of a product,do you think that mining new coins will affect the market,after all bitcoin is limited and there is no bitcoin for everyone in this world to own,the number of users are not that big and so is the reason we are seeing these sort of volatility.