Some of the common causes are

• Infection starting in a decayed tooth or gums
• Erupting wisdom tooth causing irritation to the jaw and masticatory muscles.
• Fascial space infections affecting the masticatory muscles.
• Temporomandibular Joint Disorders precipitated due to stress, injury, bad chewing habits
• TMJ Ankylosis, which is another name for the joint getting stuck because of excessive bone formation within the joint cavity.
• Oral Submucous Fibrosis. excessive fibrous formations  inside  the cheek hence restricting the ability to open the mouth. This is usually result of consumption of various irritants like tobacco, paan, areca nut and alcohol.