It's primarily due to them closing down WalMart Express, a failed concept that they tried. Supermarkets do this all the time, though they rarely let the failure get this big.
Aside from that, supermarkets shut and open new stores every year. WalMart announced that they're closing 289 stores this year.... but they opened over 400 last year. And will probably open in the hundreds again this year.
EDIT: Of the 289 stores closing, about 150 are from the USA (again, mostly WalMart Express). Looks like Brazil got hit hard too.







