Whenever bitcoin price is not changing or changing very little, it is the best time for the shitcoins to be pumped as a matter of fact! But if bitcoin price is going through a significant change (rise or fall) the altcoins usually dump.

The reason is pretty simple actually. People who trade bitcoin, usually trade altcoins as well. And traders need volatility to make profit. So when bitcoin price is not changing, they cannot make any profit either. So they start moving their coins/capital to the altcoin market to be able to make some profit there while bitcoin is stagnant. Consequently this increased liquidity helps pump shitcoins.