There were several different reasons why the Northerners became more vocal for its abolition during the 1850's.
1) Northerners were worried about the South's political power- One of the biggest reason why Northerners did not want the institution of slavery to end was because if it did, people in favor of slavery would start to outnumber the people against slavery in the US Congress. Since each state gets two senators and several representatives in the House of Representatives (depending on their population) any state that allows slavery would increase the South's political power.
2) Increase in abolitionist writings- Famous newspapers like The Liberator, written by William Lloyd Garrison, became more prevalent in the North. These newspapers inspired Northerners to change their perspective about slavery.
