Why was Kansas called the Bleeding Kansas?

Following the passage of the Kansas Nebraska Act of 1854, Abolitionists from the New England states flocked to Kansas to try and see to it that the Kansas territory entered the Union as a Free State. Meanwhile, people in Missouri were interested in seeing Kansas enter as a slave state. Both sides set up Territorial governments and claimed to be the legitimate government of the Kansas Territory. Violence soon erupted between the two sides and bloodshed became rampant. Abolitionist John Brown who later became infamous for attempting to start the Civil War early with his raid on Harpers Ferry, Virginia was responsible for the massacres of pro-slavery settlers along the Pottawatomie Creek near Osawatomie. Kansas entered the Union in January of 1861 as a Free State. Several months later the Civil War started at Fort Sumter, South Carolina. Even so, Southern sympathizers from Missouri led by William Quantrill carried out raids in Kansas and in 1863 they burned Lawrence and massacred many of its citizens.
The policy in affect that caused a problem was termed popular sovereignty.