Miners sell what they need to cover, and with the price going up, that's less BTC. They also don't have to liquidate everything. And once you've got your mining setup, your costs are largely fiat constant. The difficulty went up almost 20% since September, so miners will be less likely to get blocks or have the same share, making them less likely to sell until the price has risen enough, or they don't participate at all, and so sell nothing. This also assumes all mining and equipment costs are paid in fiat, which is becoming less and less true.