That would be my first guess, though I haven't checked to see if any of the closed locations have had union rumblings.
Note that I say "any" and not "most" or "all". One of the things that union law still doesn't have a good handle on is the firing of "troublemakers", because all a business has to do is make it look non-punitive and it's entirely legit. So basically, all they have to do is hurt a bunch of other people who have nothing to do with the union, and that makes it perfectly legal. "We had to shut down that entire department because of poor performance, and the fact that a pro-union employee worked there was just a coincidence, a tragic tragic coincidence."