People have an ideea or they already traded on dollar or euro markets. The main difference here is ....Bitcoin. Bitcoin is a crypto coin, not controlled by any government, no buffer funds and assets to back it up.
It has a value just like the other coins, given by the money that are into it. That is what gives bitcoin value.
People are treating it like dollar, or euro, or something stable....but it is not....as I said...it is not stabilized by any authority.
When people buy altcoins, not only the altcoins rise in value but also bitcoin decreases in value, as it looses money, making the effect double than on the dollar stabilized market.
Dollar or euro are not loosing so much value because people start buying like crazy something on the stock market.
On the other hand...when funds run from altcoins to bitcoin the market goes "double" as red, altcoins decreasing and bitcoin increasing....see the 2700$ peak yesterday and the 2200$ btc today. When you see a coin in "red" say -20% you have to take into account how much of that 20% fall is because of the altcoin falling and how much is because of the btc increase in value. It may as well be that the coin did not fall at all, or maybe it increased, for example if btc is up by 21% then the -20%coin is in fact 1% up.