Indeed. Remember the first rule of finance: in order for someone to sell an asset at a price, someone else must want to buy that asset at that same price. If you're selling 15,000 BTC, maybe only 200 people want to buy them for $22,000, and 10,000 people want to buy them for $20,000. And the remaining only want to spend $19,000. The market depth (the number of people willing to buy/sell for a given price) on crypto tends to be surprisingly shallow.

Selling off large amounts will push the price down, simply because you're going to run out of people buying for the high price. Then people realize you're dumping coins, and then realize since you need to get rid of them, they can buy for cheaper since you need to unload them, so why spend more? And also the psychological aspects of dumping coins (why are miners dumping unless they thought they would go down).