Mostly fear that there were among the Japanese population in the USA, people who would help Japan in its war against USA; there was fear in America of a Japanese invasion of California.
Another reason held by a few people was fear that mobs of Americans might attack Japanese residents in USA, and so, these few people thought, the Japanese needed to be put where they could be protected.
During World War I, there had been a lot of fear in America of German-Americans, but other Americans saw that they were loyal and by the time of World War II, were not so worried about them as about the Japanese-Americans. Also the Japanese looked different so that there was more fear of them (that is probably racist) and they were more likely to become a target of mob action if left unprotected, since it was easy to identify them by sight alone.
American fear and suspicion caused the internment of Japanese Americans during the Second World War. Operating out of a position of fear, paranoia, and skepticism, President Roosevelt signed an internment order that relocated all Japanese Americans and Japanese people in camps on the West Coast of the United States. The fear was that the Japanese were plotting another Pearl Harbor style of surprise attack and that Japanese spies, or those who could operate as spies, needed to be rounded up and given a loyalty oath to the nation. The camps in which they were relocated lacked effective medical care, and were situated in the desert, subject to extremely hot temperatures. The stress of being relocated and living a life in camps had adverse physical and psychological effects on many. At the same time, the consequence of the internment was that the court concluded that many of the Constitutional Rights of the detainees had been violated, under the Habeas Corpus clause of the Constitution. At the same time, I would suggest that the internment of Japanese Americans displayed a level of contradictory behavior in American policy and its ideals. A nation predicated upon individual freedom and liberty was denying it to a group, about 2/3 of whom were Americans. Finally, another consequence was that while America stood strong in its commitment to European and Japanese fascism, it was engaging in practices that were perilously close to this foreign brand of repression within its own borders.
The cause of the internment of the Japanese Americans was suspicion and fear of having the US war effort hurt. The American authorities believed that Japanese Americans were likely to remain loyal to Japan. This made the authorities think that the Japanese Americans would be likely to spy for or otherwise help the Japanese against America.
You can argue that racism was a factor since something like 110,000 Japanese Americans were interned whereas only about 1100 German Americans were interned. and The Germans were picked individually based on things they had done where the Japanese were interned based solely on ethnicity.
As for consequences, there were not really any for the nation as a whole. For the people who were interned, there were major consequences as many of them lost a lot of property when they were interned.