Walmart's plumbing problems have flushed out a potentially embarrassing labor situation.

The United Food and Commercial Workers International Union has filed a claim with the National Labor Relations Board (NLRB) that Walmart's (WMT) recent closing of five stores was done in retaliation for a history of labor activism at one of the locations, rather than because of the plumbing problems the retailer cited, The New York Times reports. The union is asking the government agency for an injunction that would require Walmart to rehire the 2,200 workers who were temporarily laid off or affected by the closings.

Since Walmart closed the five stores this month, citing plumbing problems as the cause, suspicions were aroused, especially because one shuttered location was the site of the first U.S. strike at a Walmart store. One employee at that store, located in Pico Rivera, California, told CBS Los Angeles that some co-workers believed the company was targeting employees who had spoken out against Walmart's labor practices.

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"Walmart has targeted this store because the associates have been among the most active associates around the country to improve working conditions," according to the union's claim to the NLRB, The Times reported.

The union didn't immediately return a CBS MoneyWatch request for comment, nor did Our Walmart, the labor activist group for which the union filed the claim.

The questions surrounding the store closings come at a time when Walmart is increasingly under pressure by labor activists and public policy experts, who cite its low wages as pushing working families onto relying on government aid.

In a statement emailed to CBS MoneyWatch, Walmart said, "We don't believe there is any basis for an injunction. As we have said all along, these stores were closed temporarily so we could fix the ongoing plumbing issues and it would be unfortunate if this outside group attempts to slow this process down for our associates and customers."

Each store had more than 100 plumbing problems reported during the past two years, or more than any of its 5,000 U.S. locations, the company added.

Some of the 2,200 affected employees will have the opportunity to shift to other Walmart locations. The workers are receiving pay with benefits for 60 days, and those who are eligible will be able to receive severance after 60 days if they wish, the company said.

Walmart, which is the country's largest private employer, this month boosted entry wages for current associates to at least $9 an hour, with the goal of raising that to at least $10 an hour by February 2016. While that's above the federal minimum wage of $7.25 an hour, it's still far less than the $15 hourly rate that many labor activists have been campaigning for.

The other four closed Walmart stores are in Midland and Livingston, Texas; Tulsa, Oklahoma; and Brandon, Florida.