Non Fungible Tokens are very volatile, they can easily rise or fall, there are those behind each NFT you see existing today who are helping in pumping funds into the project and once they withdrew then comes in fear because the project may lack the ability to remain operational when the giants investors are stepping down.
It's also token so the normal economic factors apply to it like demand and supply.If the project is in hype and they are building something that will provide good profits to NFT holders in the form of FP boost the prices will automatically goes up with increased demand but on the other side if they are doing nothing and just cash grab they will slowly reach zero.This is what's happening with most of the NFT projects out there.