Why was the kitchen separated from the castle?
to avoid the cooking smells
to keep the food preserved
to prevent fires in the castle
to prevent theft in the castle

Kitchen was separated from the castle TO PREVENT FIRES IN THE CASTLE.

Cooking is done in the kitchen the whole day. Therefore, fire is always present. Untended fires may cause fire in the kitchen and with the kitchen being away from the castle, the fire will not cause so much damage. Rebuilding a kitchen is easier to do than rebuilding a part of a castle.