The German DAX has had its worst year in a decade amid persistent market volatility across the globe. The stock index is down more than 18 percent since the start of the year, after it briefly fell into bear market territory earlier this month.

A bear market is when stocks see a 20 percent decline or more from a recent high — but they're also marked by overall pessimism.

According to Reuters data, the DAX is down 14 percent in the final quarter of this year and is on pace for its worst such period since the third quarter of 2011 when it lost more than 25 percent. The index is also on pace for its worst year since 2008 when it lost more than 40 percent.

Holger Schmieding, chief economist at Berenberg, told CNBC via email on Monday that the underperformance of the DAX has nothing to do with the German economy.

"It can largely be explained by three factors: Most importantly, German companies are very outward looking. As exporters of mostly highly cyclical goods such as cars and machine tools, they react more strongly to a loss of momentum in the global economic cycle," Schmieding said.

"Second, the peculiarities of the German banking sector with its myriad of non-listed public and semi-public banks and the weaker listed banks shows up in the DAX. Third, the German car sector is facing specific issues mostly related to tougher environmental standards and a belated shift to e-mobility."