Employer "retaliation" became the leading category of complaints filed by workers in 2010, according to the Equal Employment Opportunity Commission.

Of the nearly 100,000 complaints filed with the agency last year, more than 36,000 involved employers retaliating against workers who had spoken up about some perceived wrong at their company, Laurie Vasichek, an EEOC senior trial attorney, told those attending this week's annual Fredrikson & Byron employment and labor law seminar in Minneapolis.

The unusually high volume of retaliation gripes outpaced race discrimination charges, which ranked as the top EEOC complaint for 45 years.

Vasichek had more surprising news: Total EEOC complaints filed this year jumped 8 percent, to about 108,000 nationwide for the federal fiscal year that ended Sept. 30.

"That was the highest ever," she said. "We have been seeing a steady and fairly sharp increase in the number of charges that are being filed with the EEOC."

The fastest growth this year was seen in discrimination charges involving religion, disability and age.

Why the big jump in retaliation allegations?

Vasichek said that recent Supreme Court decisions made the burden of proof for retaliation claims less stringent.

"In my estimation, it is not as difficult to prove retaliation as it is some other types of discrimination," Vasichek said, adding that the case volumes are reflecting that.

As for the 2011 spike in overall complaints, she noted that some economists claim there could be a connection to the weak economy. But there do not appear to have been similar escalations in complaints during prior economic downturns.

In a bit of good news, sexual harassment cases in the workplace appear to be on the decline. In 1997, there were 80,000 total sex discrimination complaints. By fiscal 2011, the number had dropped about 29,000.

EEOC officials credited employers for clearly stated policies and a decades-long education campaign that hammered home the concept that sexual harassment is a fireable offense.