ELI5 What makes a stock fall in price?

Can someone explain to me what makes a stock go down in price from $19.80 to $16.13 which is about 20% in about 3 days? Is it selling of shares and no one buying them up? Because I thought you can only buy at a stock price if someone is selling at that stock price. Anyways please help explain if you really know. I would really like to understand why.