Why did so many northerners oppose the dred scott decision

they were concerned that laws prohibited slavery in the north might be overturned.

1.  The key military concern referenced by the Court was that "properly constituted military authorities feared an invasion of our West Coast."  The military authorities feared that Japanese Americans would give information to the Japanese or might themselves engage in attacks against US military installations.  

2.  I do not agree with the majority opinion that racial prejudice did not play a role in the US government's treatment of Japanese Americans.  There was definitely prejudice, which means pre-judging or judging in advance.  The authorities were able to force any and all persons of Japanese ancestry into internment camps, without presenting any evidence that they as individuals had, in fact, done anything to warrant such action against them.  It had been generic, stereotyped suspicion of anyone of Japanese heritage that prompted the government to restrict the civil liberties of Japanese Americans.  President Roosevelt's Executive Order 9066 (February 1942), which allowed the Secretary of War to designate certain areas as military zones, set the stage for the mass relocation of Japanese-ancestry persons to internment camps.  By June of 1942, over 100,000 Japanese Americans were sent to such internment camps.  That was a rush to judgement against thousands of persons without due process of law, to which they were entitled under the US Constitution.