With altcoins paired against btc, why price not up when btc down?

[Tekunda]

There must be a fundamental error in my way of thinking, but I admit, I don't understand why the price of altcoins which are paired against bitcoin is not going up?
In Forex, if you have a currency paired against the dollar, like GBP/USD, the pound goes up, when the USD goes down.
Why is this different in cryptocurrencies?
And just for the sake of the argument, what will happen with a crypto pair, say Litecoin/BTC should the bitcoin price go down real low, say $1000?