In the era, Taiwan had been under the reign of Japan since 1895. The people rabeled against Japanese were native inhabitants in Taiwan called Seediq. Japan had been executing a policy to "enlighten" them by preparing social infrastructure and forcing Japanese culture onto them.
The direct cause of the incident was a trouble with violence between a Japanese police and a son of the head of a tribe of Seediq. However, there was imminent flashpoint which was to spark, then the incident finally occurred, stimulated by the trouble.
Frankly speaking, I can't say "Seediq Bale" is totally an epic which has no defect as a movie. I must rather say it has many bad points. Nevertheless, I must admit I was really moved by the movie. The best thing on the movie was the speed of Seediq's running in the battle area, forest and villages. It was what only movies could realize.
The movie let me thing abount many things on history between Japan and Taiwan. For deepening your thought, I strongly recommend you watch the movie.