What is the reason for mouth ulcer after intubation?

Several reasons: Intubation can occasionally be traumatic, with injury being caused by placement or pressure from the tube or laryngoscope. Also, sometimes, after extubation, a patient can bite their tongue or lip.

Most likely trauma: Despite the great care of anesthesiologists, sometimes the tube they use can scrape the mouth and cause an ulcer. Treat the ulcer with warm saline rinses and avoid spicy or hot foods. It should heal normally.

Any trauma: Any kind of trauma can initiate the formation of a canker sore (ulcer). This "trauma" can be as a result of dental work, intubation, sunlight, stress etc.