What was one major cause of the unrest and hostility in Kansas during the mid-1850s described as bleeding Kansas by historians?

Answer 1

Stephen Douglas's optimistic idea that the people of each new state should vote whether to be slave or free.

Because these states would be voting one at a time, this gave every bully-boy in America the opportunity to descend on a single state and cause maximum mayhem.

Answer 2

Putting slavery to the vote.

The voters were intimidated by out-of-state gangs from both sides.