What: Shares of XPO Logistics Inc (NYSE:XPO) are down more than 13% today. This is just the latest in what has been a very volatile two months of trading days for the company's stock. 

So what: There isn't any material news out there about XPO Logistics today, but United Parcel Service, Inc. (NYSE:UPS) released its third-quarter earnings today, with revenue coming up short of estimates, and actually down from last year. The decline in revenue looks to have caught Mr. Market a bit flat-footed, but UPS said that currency exchange was a big driver behind the revenue shortfall -- a common refrain from U.S. multinationals over the past year. Factoring that out, sales would have increased by almost 2%. 

UPS management also said that fuel surcharges -- payments it gets from shipping customers to cover fuel costs -- declined. That's not surprising, considering that diesel and jet fuel prices have fallen nearly 40% over the past year. UPS shares are down more than 3% today. 

Now what: With a dearth of news out there about XPO Logistics, UPS earnings is the most likely "culprit" behind the sharp drop in its stock price. OK, the real culprit is the incredibly high percentage of XPO Logistics shares sold short right now:

XPO Short Interest data by YCharts.

As of the most recent data, more than one-quarter of shares outstanding are sold short. When this percentage of a company's share float is held short, it can really ramp up volatility. As you can see in the chart below, the stock price really began falling a few months back, corresponding with the sharp increase in short-selling:

XPO data by YCharts.

But don't mistake causation with correlation. In other words, the short-selling itself was a product of a significant shift in market sentiment when the company began shifting away from its "asset-light" model, and began making substantial acquisitions of shipping and trucking operators. 

The point? XPO Logistics was a hugely successful investment operating in a very different way from the kind of business CEO Brad Jacobs is now building, acquiring other operators and taking on a lot of debt. And because of this nearly diametrically opposed strategy shift, a lot of people are betting against the company being successful -- at least in the short-term. 

Until Jacobs' bold moves to rebuild XPO Logistics into a more integrated company begin bearing fruit, there's likely to remain a high level of short interest, and that means more wild swings like today's, and often when there's not really any material news about the company. 

Jason Hall has no position in any stocks mentioned. The Motley Fool recommends United Parcel Service and XPO Logistics. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.