It was using pooled token mechanism. It's a bit different compared with the centralized system. This caused by when someone was adding more liquidity by buying the tokens that already pooled and this will increase the price and otherwise people who swap their tokens for native tokens that already paired will make the price to go down.
You can try to read some articles and you will be explained so well about that. I meant about the liquidity pool. the dex was using this mechanism and this is not the same like when you are trading on the centralized exchange site.