If we based on many prediction, Yes it possible to happen and it will happen soon because according to the prediction on "experts" bitcoin will fall before the year 2019 ends, and now bitcoin experienced a continue price falling and when the time that bitcoin will fall they say that the altcoin will increase. So if we based on that theory the alt season is near to happen.
Yes, the alt season will come when the bitcoin dominance will decrease but at the same time we do not want the overall market cap to decrease too. If the marketcap of crypto remain stable only then the money will flow from bitcoins to the altcoins. However if the marketcap also decrease with the bitcoin price decrease, then the alts will dump too.