I agree many altcoins are probably highly overvalued. Some of the increase is a growing number of people seeing the potential in something like Ethereum, who then buy, and the price goes up, and there is media attention, so more people find out about it, and buy in, driving the price up some more. Ethereum also had a great few months of news, which contributed to that growth cycle, and media outlets like Bloomberg are talking about it almost every day now. There is also a lag between when someone finds out about it, decides to get in, and can actually buy some, due to verification and wire transfer times. Sometimes it can take 3 or more weeks for the average person to get through all of those steps, so any good news in crypto takes a few months to fully affect the price.
As for altcoins, I think it's a lot of people not wanting to miss on the next Ethereum. Once the market realizes that only a handful of cryptocurrencies really have a future, there will be a big crash for sure. I could be wrong but I see Bitcoin kind of like Myspace, and Ethereum like Facebook. And there won't be much room for multiple Ethereums. It's possible another coin could show up to take Ethereum's place, but Ethereum does enough things right and has a big head start that I think it will ultimately be the market leader. Before we get to the crash though, there is money to be made riding the bubbles in the short term.