For some investors, it feels a lot more like 1999 than 2013. Third-quarter earnings have not been spectacular. The U.S. isn't expected to grow at anything close to breakneck speed next year. And there are few industries experiencing huge profit expansions. Despite all that, a number of high-profile—mostly technology—stocks are soaring. The heady advances are making shareholders of these companies big money, but they're also raising serious questions among some analysts about whether the unusual trading is a troublesome sign for the overall market. Last week, the Dow Jones Industrial Average set a record on Tuesday, but closed up just...