It is not really that hard to decipher. Virtually, 80% of altcoins are trading pairs with bitcoins. What this means is that, when bitcoin pumps, traders will close their positions and move most of their portfolios to bitcoins while alts dumps.

When bitcoin is in a downtrend, traders out of sentiments  close their BTC/Alts trading position and quickly move to tether or any other stable coin, to avoid being in BTC as it loses value, while alts still dump.

Bitcoin dictates the dynamics of many of these altcoins because altcoins are often traded against it. That's why whenever btc is stable, alts can now move at its own pace.