It's actually very difficult to differentiate between onion maggot and onion smut damage from this picture. In order to determine the cause you must examine the affected seedlings. Onion smut is a fungal disease that forms black lesions within the leaf tissue. If you pull up a seedling and see black colouration, you are dealing with smut. On the other hand, if you don't see any black and seedlings have toppled over, you're probably dealing with onion maggot. You may also see maggots on the damaged seedlings.