When BTC dominance is high, it affects the alts then after BTC dips, its the alts turn to pump. Think of it as a circulation of money. This is because this is the money flow as follows:

1. Fiat (e.g. USD, EUR, GBP)

2. BTC

3. Large Cap Altcoins (e.g $ETH)

4. Mid Cap Altcoins

5. Low Cap Altcoins

6. BTC again

7. Fiat