This event is referred to as Bleeding Kansas and took place when Kansas was voting on whether or not to be a slave state.
Kansas had been granted popular sovereignty meaning they were allowed to vote on whether or not the state would be a slave or free state. Abolitionists flooded the state to encourage citizens of Kansas to vote to be a free state. However, Missouri being a slave state was concerned about being located next to a free state, so people from Missouri crossed the border and attempted to vote for slave status. While there, they got into large riots with abolitionists hence the name "Bleeding Kansas".
