Why miners? There are a lot of people with a lot of bitcoins who are not miners.
Good question!
Existing coin holders might miss the chance and sold too much coins at a relatively low price, thus they don't have enough coin to sell at higher price. But miners can sell coins at any price, since they have a continuous inflow of coin income
What you said is true, there might be large players with hundreds of thousands of coins, manipulating the price, but how effective a single player can do is a question
