I guess some Amazon sellers sometimes send out free stuff randomly with the intent to use the shipment to generate a fake review, or to lower their ratio of returned items. They call it "brushing".
Seems a bit silly to me: if someone is in a position to post fake Amazon reviews in your name, shouldn't they be able to do a lot more damage? But, I suppose you would report a fraudulent order right away, but might not even notice a fraudulent review