Why have Wal-Mart suddenly closed 5 stores without notice?

Conspiracy theorists are having a field day at the news that retail chain Wal-Mart suddenly closed 5 of their stores on Monday evening.
Wal-Mart say that the closures may last up to six months and will affect around 2,200 workers in Texas, California, Oklahoma and Florida.
Employees were given just 2 hours notice before the closures, with the official reason given as ‘plumbing problems’.
This would be understandable if it was just one store in one state, but the fact that this has taken place in multiple locations at the same time just doesn’t add up.
To make things even stranger, the stores are all fully stocked with supplies (many perishable), which haven’t been removed, and are now guarded by armed police.
One commenter on a popular conspiracy forum said: –

I can personally confirm the Tulsa store WAS closed with 3 hours notice and as far as I know it is still being guarded by some type of police force who are armed and working in pairs. The front is covered up in black plastic.

A plumbing engineer for the chain stated that the official explanation doesn’t make any sense, telling WFLA: –

Even if they had to replace the whole sewer line, it wouldn’t take six months to replace a whole sewer line in that store

Some employees are speculating that the stores have been closed due to worker protests for higher pay, while others on internet forums are suggesting there may be something more sinister at play: –

Getting ready for Martial Law practices or maybe something worse!!

We’re not sure we would go that far, but there is something seriously odd about this story. Why the need for armed guards? Can 5 stores suddenly develop plumbing problems at the same time?
We’ll give you an update as soon as we have any more information.
What do you think is going on at Wal-Mart?