Miners will always need to sell their coins to pay for expenses, but I'm not sure the "whales" that have been dumping their coins at below market rates could possibly be miners, because that's such a dumb move.
I haven't followed it that closely, but to me it appeared that the big dumps, especially the one at $300 recently, were efforts to manipulate the market (down) at a loss to whoever did it. Akin to what happened with Gold / GLD a while back where someone apparently took a huge loss just to dump a lot of Gold at below market rates, driving the price way down.
Imagine, that you had 30 000 bitcoins bought for 10$ each(or for less). After the high last year the price is falling all the time and in 4. of October it reaches a new low in this year. Now, if you sell at 300$ you have a 3000%. You get 5Mio. dollars after taxes for sure. You can park that money on a bank and get 50 000$ interest on it or more. For the rest of your life. Or you buy stocks, real estate, gold etc. and get a profit out of it, that guarantees you a good income for your life time.
So. There is a chance that the Bitcoin price will be at sub 10 dollar in a year. Maybe it will go to zero in a few year.
Would you sell or risk it?