My stored onions are rotting and spreading a bad smell, what went wrong?

Soft, slimy, foul-smelling onions are usually a bacterial or fungal rot, and once it starts it spreads bulb to bulb through contact and moisture, so quick action matters. The root causes are usually curing and storage problems: onions that were not cured (dried) properly after harvest, bulbs with thick or green necks, mechanical bruising, or storage that is too humid and poorly ventilated. To limit the loss now, sort through the lot and remove every rotting and soft bulb immediately, because one rotting onion infects its neighbours; do not just leave them in. Improve airflow by spreading bulbs in thinner layers, opening up the store, and keeping it dry and shaded. For the next crop, focus on prevention: harvest at the right maturity when tops fall, cure the bulbs well in the field and shade so the necks dry and the outer skin sets, discard thick-necked and damaged bulbs before storage, and store in a ventilated structure where air moves freely, not in closed damp heaps. Storage onion varieties also keep far better than soft kharif types. Verify good curing and storage practice for your variety with your KVK or local horticulture office.