as altcoin demand increases. investors are selling their bitcoins to buy altcoins to make a profit. so bitcoin prices can go down.
Around this time of the year, similar events have happened. ICOs were in heat that time and altcoins in general are gaining more compared to bitcoin. Although, in reality, bitcoin's price is pegged to all alts. So whenever bitcoin's price goes up, alts do too, vice versa.
this is some of the movement thing that are really or most common happen on this crypto we should analyze the fact that bitcoin and altcoin on how does the other move for then they have to make some also.