Which cause for alarm led japan tokugawa shoguns toward isolation
the influx of foreign traders
the arrival of dutch learning
the rapid spread of christianity
the attempted mongol invasion

The rapid spread of Christianity.  Spanish and Portugese missionaries made their way to Japan in 17th Century.  In time, they manage to convert many of the Japanese into the Christian faith.  The Japanese lords were alarmed by the growing spread of Christianity as the influence of the missionaries was growing on the people.  They started to view the missionaries as predecessors of  military colonizers and started to be hostile against the missionaries. In 1627, a revolt by Christian samurais and peasants called the Shimabara Rebellion angered the Shogunate and issued the Seclusion Decree that outlawed Christianity and closed Japan to the world starting a period of isolation.

The answer is ethnic groups.