It just means that due to highly volatile and sensitive condition of the market currently, people are shifting to much secure investment that is BTC. Once the market gains stability and moves towards bullish trend, altcoins will again gain dominance in the market. Although according to predictions of many people, 75% of the altcoins will fail. So its wise to only invest in well established coins that hold real world applications. BTW BTC has already reached 53.4% mark in terms dominance.