you can read up on the POS (proof of stake) algorithm.
It is designed to reward people that hold (stake) a certain amount of altcoins for a certain time.
This way, you can create passive income, however, it's only a limited number of altcoins that use POS, if you pick an altcoin to stake, and you gain 10% in one year, but in the same time, the altcoin's price goes down 50%, you end up with more altcoins but with less actual value.
You probably mean the Ethereum. In these coins the developers laid down the mechanism of inflation. POS is an attempt to stop this process but I'm sure it's a ticking time bomb. When it explodes all will suffer who keeps ETH. I do not recommend long to keep money in altcoins.
the reason why the price of PoS coins go down is because they are practically doing an airdrop but more frequently than a normal airdrop.
people are known to dump what they get for free. there are a lot of different reasons for that too. some fear the price is going to go down because some others will dump before him, some want to just sell the free coins to have a "passive income" like OP so they sell,...
but for whatever reason the sells take place and they create a pressure on price consistently and in long term. which ends up reducing the price slowly but surely...