If you said so then its a good indicator that every middle of the year the price is cheap, but let me tell you something that its random and not every coins will be cheap at the middle of the year.
There are many reason why the altcoins price seems down, maybe in the market they have a problem or maybe they depend in the bitcoin. We know that the bitcoin is the main coin and the other coins only based on the price of the bitcoin so when the price of bitcoin goes up then the other coins rising too. And when the price of bitcoin going down then the other coins falling.
Many people are sure that Bitcoin price affects the value of altcoins, and this is true. Additionally, the news about the hacker attacks, bans of ICO ads and prohibition of the cryptocurrency in several countries decrease the coins' price.