But altcoin price is quoted in btc, so the btc price reduction in terms of fiat is already factored in. Unless you mean that a reduction in faith in btc leads to an even bigger similar effect in all altcoins.

But really the reason is just that all those silly alts were made tradeable way too soon. Supply has gone massively up. Now people are trying to consolidate to whatever coins still have credibility. Thats why litecoin seems to be holding on to its usual value relative to btc.

Then there is the issue with exchanges lately not being exactly trustworthy, so many people are just "cashing out" in btc and ltc proper wallets to ride the storm out