Are price movements of Bitcoin and altcoins inversely coupled? It is said that when the value of Bitcoin rises altcoins lose value, or at least their price in BTC falls, and I would like to understand exactly why.

If I have spend $10 in BTC to buy BLK, and BLK doubles its BTC price, but in the meanwhile the BTC value rises 6%, too (in reference to USD), I have $10 * 2 * 1.06 = $21.20, so, there's no loss of value.

However, on yobit at least, there was altcoins which saw a generalized selling, and other altcoins that just saw a tiny activity. In other words, the market was mostly paralyzed or only selling. Why is that?
The idea that bitcoin rising leads to altcoins falling is an unproven hypothesis, it is something that traders have noticed, but due to the various flaws in human observation, we cannot say for certain it is true.

That said, there are several factors that could lead to this effect in the short term - but these are all speculative.


Traders are attracted to liquidity. Bitcoin has the most liquid leveraged markets of any crypto currency, or at least historically that has been the case.  While the relative swings of bitcoin are smaller than for altcoins, the use of leverage allows traders to create large profits off of those swings in highly liquid, leveraged environments. When bitcoin is climbing rapidly, traders might be incentivized to reduce altcoin positions in order to increase their available bitcoin capital for leveraged trading.
Arbitrage against US dollar. Many altcoins are traded against the US dollar as well as against bitcoin. Let's say ETH is $50 USD, and .05 BTC/unit, and bitcoin is $1000. Bitcoin suddenly spikes to $2000 - the standing order books for the BTC/ETH pair will still show a price of .05, but that .05 is now worth $100. An arbitrage trader can buy ETH on the USD market and sell it on the BTC market for a near-instant profit - this drives down the BTC value and drives up the USD value until equilibrium is reached.
Bandwagon effect. The idea has become axiomatic so when bitcoin spikes, traders may be psychologically primed to sell altcoins in response.


Note: in many situations these factors are overwhelmed by other market forces, and it is not entirely uncommon for altcoins to outpace bitcoin during sustained price increases - this is demonstrably the case for most of the early 2017 bull market.