Marijuana plants can be induced to flower early by a variety of factors, including changes in light cycles, stress, and genetics. Shortening the amount of light the plants receive each day can trigger the flowering process, as can exposing them to prolonged periods of darkness. Stress, such as damage to the plant or changes in temperature or humidity, can also cause early flowering. Finally, some strains of marijuana are genetically predisposed to flower earlier than others, regardless of environmental factors. 
Understanding these factors can help growers manipulate their plants to produce flowers on their desired timeline.