Altcoins are more sensitive than mainstream. When there is a surge, if altcoins drop while mainstream slowly rises, then caution is needed. This may indicate that the surge has ended and is entering a consolidation or decline period.

Altcoins have smaller market caps, so when the main players take profits, it becomes evident, while the overall market moves more slowly. This often leads to situations where people go long on altcoins when BTC is rising, but the altcoins end up falling. A group of people just doesn't understand why altcoins aren't rising.