There are many causes for acquired or congenital microstomia. The most common acquired causes include orofacial burns, resection of lip masses/stumors and the CREST syndrome (calcinosis cutis, Reynaud phenomenon, esophageal dysmotility, sclertodactyly, and telangectasias).

Certain genetic conditions associated with microstomia include Freeman-Sheldon syndrome (ie, craniocarpotarsal dysplasia, whistling baby syndrome). Other disorders that may cause microstomia include Hallermann-Streiff syndrome, oro-palatal dysplasia, Fine-Lubinsky syndrome, restrictive dermopathy, types of epidermolysis bullosa, and, occasionally, Down syndrome and hemifacial microsomia.

To my knowledge, the ulcers in Behcet disease are not associated with the development of
microstomia.

This Emedicine website about microstomia may be helpful: http://emedicine.medscape.com/article/878332-overview