Perhaps the most important is the number of hackers who stole Ethereum from several exchanges and immediately sold it because it was as if investors were worried to invest in crypto currency because without any security guarantees, other factors might be because the statement from Vitalik Buterin announced the resignation of core team Ethereum
It is difficult to say which price can be the lowest one for Ethereum. I guess the altcoin can continue falling in December. After the winter holidays, when the New Year and Xmas fuss is over, it will start growing.