If it's just one ear on one sheep, it could have been an injury or sting started it off. Or it could be an advance notice of a mineral deficiency - I think it's cobalt deficiency when it's scabby ears, but we pretty much always give cobalt and selenium if they need anything, and copper once a year (max) and only if we are sure they aren't sensitive to it.
If a group get scabby ears in summer and have access to boggy type ground, or to overgrown weedy meadows, it could be they've rubbed against a plant which causes photosensitivity (such as bog asphodel or hogweed.)