When BTC rises sharply and altcoins rise slightly, reinforcements arrive.
BTC is stable, altcoins rise randomly, dispersing forces.
BTC rises alone, altcoins fall, concentrated attacks.
On the contrary...
BTC falls sharply, altcoins fall slightly, funds withdraw.
BTC falls slightly, altcoins plummet, shrinking positions.
BTC stops falling, altcoins collapse, and the battle ends.