The return of FOMO is fueling concern that the market may take a tumble.

Strong earnings and an overall downward trend in inflation have helped fuel a surge in the S&P 500 that few would want to miss. But with much of the index’s 30% rise over the past 52 weeks stemming from just a handful of stocks like artificial-intelligence play Nvidia Corp., some market watchers fear there may be a melt-up unfolding.