This started happening back when those exchanges switched to zero fees. In fact, some of the bots there will ONLY trade with themselves, meaning they cannot control the price 24/7. I also believe those exchanges are used to manipulate price. Why they do what they do and when they do it, I will not get into.

However, they are not the only ones with loads of coins, so it does not always work in their favor. Whales only move markets until a bigger whale comes along.