Miners don't "dump" value of Bitcoin by putting too large sell orders, quite the opposite... They try to drive the value higher so they get same amount of FIAT... But because relative value of FIAT has gone down, they don't need to put as large sell orders as when FIAT was stronger.

It's the other traders (people who don't or can't mine) who cause the massive volatility of the Bitcoin as they try to ride the ups and downs and are too impatient to wait for the high and low points.