What happened

Investors reacted negatively to Ballard Power Systems' (NASDAQ:BLDP) third-quarter earnings report last month -- or more precisely, to the company reporting a lack of earnings, and losses instead. But what a difference a couple of weeks make.

Since the month of November began, Ballard stock has enjoyed an astonishing 31.5% run-up in share price. Investors approved of Ballard CEO Randy MacEwen's prediction of "continued strong support by the China national government for the adoption of fuel cell vehicles and hydrogen fueling infrastructure, including a subsidy program we expect to extend through 2025," which would benefit Ballard's business.

Up until yeserday, that is. On Tuesday, Ballard's share price tumbled, and closed the day down 10.5%.

A blue fuel cell

Image source: Getty Images.

So what

What spooked investors today? That's hard to say. There is no stock-specific news out today to explain the drop, no downgrades (or upgrades) nor anything huge on the macroeconomic front. The S&P 500 as a whole closed the day basically flat.

My guess: This is a simple case of investors "taking profits" after the terrific run-up in Ballard's stock price.

Now what

And if that's the case, then there's no reason to expect an immediate bounce-back at Ballard. There are, after all, still a lot of post-earnings profits left to take. This sell-off could get worse before it gets better.