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Many times a dog will display this behavior because they have gotten pollen or another substance on their fur and they attempt to rub it off. In some cases, they are just itchy and rub on an object that they know they can get to places they can't with their paws. We frequently see dogs rubbing their faces on the carpet and furniture. Some owners will use baby wipes to wipe down their dogs coat when they come inside during heavy pollen times. It can actually help if a dog has skin issues due to pollens.
If she is on the couch a lot, then she might just be rubbing her scent all over the couch as her way of claiming it as her own. If you do notice that she tends to scratch more often after coming inside, you might try benadryl and see if that helps. Benadry can be given at a dosage of 1mg per pound every 8 hours. Try giving it before you take your walk and see if that stops the behavior. My dogs are not allowed on the furniture but will come in and roll on their backs and wiggle back and forth on the carpet to scratch their own backs.
As long as she isn't continually scratching and doesn't have hair loss, it probably isn't anything to worry about. I hope this information is helpful to you.