Well one of the major reason is due to the price in btc
As all the crypto are based value in satoshi
So usually if you see all greeen, you could expect the coins value to move together
Example
100,000,000 Satoshi Price = $1000
X cost 1,000,000 = $10
If 1 btc moved 10%
It will be $1100
Hence, X will move 10% since it is value in.satoshi
This is the correct explanation, you can see it pretty clearly on coinmarketcap, the graphs of the btc prices show almost similar movement.
You do also see some outliers on there, especially among the newer coins.