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iOS 5 introduced two new ways of wireless backups, iCloud and Wi-Fi Sync. This obviously meant that Apple had to work on the size of backups of their in-built apps as well as third party apps.
To do so, iOS at times clears cache and temporary files stored by certain apps, and while doing so it changes the label under the app icon to "Cleaning."
Rene Ritchie over at TiPB explains: