The riddle
Why Did the Cranberries Turn Red?
Which dressing is meant
Three meanings of dressing matter here, and Wiktionary lists all of them: the filling put inside poultry, a sauce for salads, and the activity of getting dressed. On a holiday table the first one is the likely guess, since stuffing sits next to the turkey. The riddle needs the third. The cranberries are pictured as little onlookers who catch the turkey getting dressed, and they react the way a person does when embarrassed. Both senses are on the table at once, which is why the line works.
Red from a blush and red from ripening
Faces turn red when a person is embarrassed, and the joke gives that reaction to the fruit. The real reason is plainer. Wikipedia's article on the cranberry says the fruit starts light green and turns red as it ripens. Berries that get sun become a deep red, while those that never fully mature stay pale pink or white. A cranberry that has not changed color is simply not ready yet.
A crane in the name
The word comes from Middle Low German kraanbere, meaning craneberry. The same article explains that New England colonists thought the plant's flower looked like a crane, with the stem like a neck and the petals like a head and bill. Few people picture a bird when they open a can of sauce, but the bird is still in the name.
Berries that float
Many cranberries are harvested wet. The bed is flooded with 15 to 20 centimeters (6 to 8 inches) of water above the vines, a harvester removes the fruit, and the berries float to the surface, where they are corralled and pumped out. In 2021 Wisconsin produced 59 percent of the 360,000 metric tons grown in the United States, according to the same article. For another riddle that gives a piece of food a human reaction, read why did the banana go to the doctor.