Why did northerners support runaway slaves?

Answer 1

Because they reacted badly to the Fugitive Slave Act, which turned every citizen into an unpaid slave-catcher. And they had read 'Uncle Tom's Cabin', which was written as a protest against it. That novel had drawn attention to the Underground Railroad, the safe-house system that smuggled runawayslaves into Canada.

Answer 2

It required northerners to help recapture runaway slaves which was disliked either on moral grounds as it was for abolitionists but also in some reasons of fear of a Slave Power Conspiracy with the Southern Slaves States holding greater power over Congress than the North, many northerners in the mid 1800s believed many of the politicians in power were under the thumb of Southerners. This act in particular placed fines on people who would not cooperate and jail terms on people who helped fugitives escape. Also, southern slave catchers (aka federal marshals) roamed the North, sometimes capturing free African Americans with little regard for if they were free or fugitive slaves.