The value of Steemit (a cryptocurrency) compared to US dollars (a fiat currency) goes up and down throughout the day because all over the world, people are constantly trading millions of dollars back and forth.

EXAMPLE
If somebody suddenly sells, say, a billion dollars worth of Bitcoins for a much lower than average price, everybody else will probably have to lower their cryptocurrency selling prices in response.

We minnows don't need to worry about such things.