What causes a canker sore?

It is unknown. ...but some studies had shown it is triggered by changes in temperature, hormones, trauma, allergy,...etc. .
Not well nderstood. Canker sores are not well understood. Causes are trauma, vitamin deficiency, hormonal, autoimmune, hormonal, allergic reaction.
Canker sores. Canker sores can be caused by a number of things, toothpaste additives, autoimmune attack, smoking, stress, hormones, allergies etc.
Idiopathic condition. No one knows what causes canker sores. They may be caused by a virus, may be a form of allergic reaction or may result from an auto-immune condition. Nothing cures canker sores or prevents recurrence.