It's not because of PoS. Ethereum is just recovering from a bear market in BTC/ETH pair. Look at the orange line https://coinmarketcap.com/currencies/ethereum/.
What is PoS?
Depends greatly on context, but here it refers to Proof of Stake.
In relation to price, PoS removes the energy expenditure required to mine coins. Without constant selling pressure of miners selling to cover electricity costs, price may rise.
Proof of Stake. [1]
Ethereum as an example, is near an all time high as PoS nears. Many other tokens with unique use cases like NEO and XMR are at all time highs as well.