Why and how did some northern state legislatures reject the federal governments passage of the fugitive slave act?

They didn't ignore them. They reacted sharply against them.Under the new law, members of the public were obliged to report anyone who looked as though they might be a runaway slave, on pain of a heavy fine.This offended large numbers who had not felt strongly about the slavery question before, and it recruited many new people to the Abolitionist cause. This increased the numbers who helped to organise the Underground Railroad, the safe-house network which smuggled fugitives into Canada.