Some southerners threatened to secede over California's application for statehood during the middle of the 19th century because California would enter the US as a free state.
This means the state would not have slavery. Along with this, it would mean that there would be more representatives in Congress (aka the House of Representatives and the Senate) that are free states than slave states. This situation puts the southerners at risk, as the free states could make laws that would get rid of the institution of slavery. At this time, almost the entire south's economy depended on slavery.
