For once more, you speak out the word of logic. The voice in the back of our heads that tells us: "Is X coin that much valuable that deserves a 10 times increase in just 2 days? Does this market cap really represent the issues that this X coin tries to solve, or the new benefits it provides to humanity in this period of time? Does this new alliance/team formation with company X or the new listing in an exchange platform make the coin so much more valuable?" We need to ask all these questions before we place any bets, and more importantly before we miss lead this wonderful technology into a rapid fall.
While I do believe that there are a number of reasons for the existence of about the top 200+- crypto currencies, I am afraid that at the moment most of the market cap is represented by people that have absolutely no clue in what they buy. People that are hoping to catch the next 10x and make huge profits. Don't get me wrong, I my self consider a "wise" strategy to invest 10-20% of a portfolio in many different alt coins, due to the increasing potential of huge incomes, but please please please: always do some basic, 30 minutes research, to find out why and who developed any of the coins in your portfolio.
A final thought on how this could lead to a sudden fall: big players with lots of money invested in this market not only follow the news, but actually deeply understand the uses and practices of each coin. While it is very hard to actually count that into a number (price), it is much easier to get a feeling of an approximate evaluation. So when these people understand that any crypto is very overpriced, they will cash out lots of money and create (willingly or unwillingly) panic which will lead in an even bigger decrease in market cap. Then, as soon as the price returns to the level that represents the real value, they will buy them back.
I don't know if we got a week, or a month, but I feel that we are getting closer and closer. If this happens, I believe that market cap of cryptos will drop 30-40% and bitcoin will rise again to 50% market dominance; thus, maintaining the same price or small increase. I am NOT a financial advisor, and you should all make your own research before you take action into investing.