It's all based on psychology.

During a btc bubble, alt/btc tends to move against btc. This is because
1) Traders all want to hold btc to be part of the btc rally, and are willing to sell the alt at any price, to oversold levels, to do so. This drives alt/btc down.
2. Traders have the attitude that alt/usd is a constant and then when btc/usd rises, mathematically, alt/btc must fall, and vice versa.
3) They use the alt as a hedge against btc corrections during the rally.
Summary: Traders want to hold BTC.

During a btc crash after a bubble, alt/btc tends to move with btc. This is because
1). Investors are so bearish that they sell the alt to oversold levels because it necessary to sell btc for fiat.
2) A bearish mindset that if btc can crash then alts must be completely worthless.
Summary: Traders want to hold FIAT.

When these conditions are not met, often btc/usd does not affect alt/btc. This is because
1) Investors are not having trading reactions on one pair based on the other pair and there is no psychological reason to do this.
2) Investors realize that the value of alts rise and fall with the value of btc.
Summary: Theres no strong influence over what traders are holding.