What happened

Shares of Boston Beer (NYSE:SAM) fell last month, dropping 17% compared to a 2% increase in the S&P 500, according to S&P Global Market Intelligence. The slump only erased a portion of investors' recent returns, and the consumer staples stock remains higher by over 50% so far in 2019 compared to an 18% rise in the S&P 500.

So what

September's decline came as investors wrestled with the potential for new competitors to snatch away some of Boston Beer's recent market share gains. Specifically, the hard seltzer market is awash in popular brands such as White Claw, which appears to be winning converts from Boston Beer's Truly franchise.

Two men clinking their beer bottles together and smiling

Image source: Getty Images.

Now what

Investors will have to wait until Boston Beer's next quarterly report -- likely in late October -- to see if growth is slowing. Its Q2 announcement showed no signs of competitive intrusion, with the Truly brand powering a 17% spike in depletions, a measure of consumption.

It's unlikely that Boston Beer will see that impressive momentum arrested in just one quarter. Yet investors are right to worry about the sustainability of revenue growth, given that so much of it has come from recent launches like Truly and Twisted Tea.