What's happening: Natural gas-for-transportation refueling leader Clean Energy Fuels(NASDAQ:CLNE) stock surged 10.5% today, on moderately high trading volume that was about 25% higher than the average over the past few months. 

CLNE Price Chart

CLNE Price data by YCharts

Before today's big jump, Clean Energy's stock has plummeted since the beginning of May, falling by almost half, after doubling in a huge run-up from the beginning of the year. After all the noise, the stock is up about 15% this year:

CLNE Chart

CLNE data by YCharts.

Why it's happening: In short, it's shorts. Or, at least, it's likely that much of the price volatility is a product of short-sellers exiting the stock in late April, as well as short-term-focused traders moving in and out of the stock -- as you can see in the chart below, which shows trading volume along with the daily stock price:

CLNE Chart

CLNE data by YCharts.

One of the key drivers behind all that trading in late February and early May? Earnings reports that the market has taken as relatively positive -- or at least short-sellers took as reason enough to move on. 

In each of the past two quarters, Clean Energy continued the strong trend of fuel sales growth, and has managed to increase delivered volumes more than 20% year over year, even as cheap oil prices make natural gas' cost advantage over gasoline and diesel -- one of the company's biggest selling points -- less attractive. 

Back to the short-sellers, it's worth noting that the percentage of Clean Energy shares held short is on the rise again, as of the most recent data available:

CLNE Percent of Shares Outstanding Short Chart

CLNE Percent of Shares Outstanding Short data by YCharts.

This is more evidence that today's sharp price jump was at least partly driven by short-sellers choosing to exit their positions, especially considering that there wasn't any material news about the company today, or even any positive macroeconomic news to point to. 

In summary, it's another reminder that sometimes stocks just move up and down because of what people do, and not because of anything related to the company itself. Furthermore, there's not really any solid way to consistently use this information to inform what the stock will do tomorrow, or in coming weeks. We can guess, but that's really all we can do. 

Instead, it's probably better to keep an eye on the long-term prospects.