If their body temp is warm, it can cause the ears to droop due to the increased blood supply in the ears, which can still happen indoors, especially if moved from outdoors to in and the rabbit still has a thick winter coat. Or even rabbits normally kept indoors, can still get too warm, even when the indoor temp is normal(70 f). There is a mention of it sometimes being caused by nutritional deficiencies, from low vitamin A/E in growing rabbits, due to it affecting cartilage formation.
If it clears up the same day, I would say probably the bunny was just too warm. Maybe offer cool water to drink, a cold tile to lay on, and/or a bottle of frozen water to lay next to, and see if that helps. If it's consistent and continues with no relation to your rabbits body temperature, it could possibly be a vit A/E deficiency from an issue with the rabbits feed pellets. Pellets that have been sitting on the shelf too long at the feed store, then at home, can affect the vitamin content.
Vitamin E Deficiency
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WARNING: CONTAINS GRAPHIC MEDICAL RELATED PHOTOS)
Vitamin A toxicity and vitamin E deficiency in a rabbit colony
If the whole ear was drooping, ear injury or infection could be a cause.
