What's happening: Shares of natural gas refueling expert Clean Energy Fuels Corp. (NASDAQ:CLNE) are up about 8% as of this writing, having cooled off a little. Earlier in trading, the shares were up about 11%. 

Why it's happening: It's been a whole week since Clean Energy stock moved by 10%. A whole week, people! It was due. As much as that's in jest, it's more or less been true so far this month:

CLNE Chart

CLNE data by YCharts.

But as to the why, in short, it's shorts. Through the end of June almost 18% of Clean Energy shares were sold short. While this represents a decline from earlier in the year and from prior years, it's still an incredibly high percentage of short activity, and with earnings set for Aug. 5, there's actually the chance that short interest has increased in July. 

Either way, this much short activity significantly adds to a stock's volatility. When shorts "open" a short position, they are literally selling shares they don't own, using debt or some form of collateral to cover their obligation, and creating a negative holding in that position. If the stock price falls -- as it did by 5% yesterday -- short sellers who want to cash in must buy those shares back to exit the position. If a lot of short sellers exit at the same time, which can happen following a sharp decline like yesterday, all of those buy orders can create a squeeze, pushing up the bid price as short sellers race to the exits. This can be even more extreme around earnings season, when trading activity in general tends to be much higher. 

At any rate, for long-term investors, there's nothing going on -- just more volatility, likely as a result of short sellers exiting the stock, as it's still down from its recent highs. With earnings only two weeks away, there's probably more volatility ahead.