Hay can ignite without a spark
Bales stored with too much moisture generate heat as they cure, and in the wrong conditions that process reaches ignition on its own. It is well documented, it is the reason barn fires happen without an obvious cause, and it is why carriers ask where feed is stored and how much.
Separation is the control that matters
Storing hay in its own structure, away from the house and away from equipment and fuel, is what limits a fire to one building. Carriers rate the distance between structures for exactly this reason, and a feed barn attached to a garage is a materially different risk from a detached stack.
The stored feed itself is rarely well covered
Hay and feed are farm property rather than household contents, and a homeowners form covers them barely or not at all. On a property carrying a season of feed that is a real value sitting outside the policy unless somebody arranged otherwise.