It happened before because there were people who intentionally manipulated the prices, trading their coins directly to those altcoins. The previous dip on bitcoin price was not a manipulation but rather an effect from people who sold their coin directly to their fiat currency. I remembered seeing almost all of the currency having red marks during the correction. We need to accept that there are investors who set their ceiling price at $10,000 and when bitcoin reached it they sold their coin. It may not be considered as panic selling but it really affected the market since bitcoin's price is only based on demands from investors.