Final answer:
Preston Brooks attacked Charles Sumner due to Sumner's speech against the Kansas-Nebraska Act, his attack on Senator Andrew Butler, and in defense of southern honor, leading to the incident known as Bleeding Sumner.
Step-by-step explanation:
Representative Preston Brooks attacked Senator Charles Sumner in the Senate chamber because of Sumner's vehement opposition to the Kansas-Nebraska Act and his support for Kansas to enter the Union as a free state. Sumner's "Crime against Kansas Speech" harshly criticized the push for slavery in Kansas, attacking the integrity of pro-slavery legislators, including a personal attack on Senator Andrew Butler, who was a relative of Preston Brooks.
In response, Brooks assaulted Sumner with a cane in a defense of personal and southern honor, an incident that became known as Bleeding Sumner. During the debates on Kansas statehood, tensions were incredibly high as neither a proslavery nor an antislavery constitution would be likely to win approval due to the split control of Congress.
However, political stake of the issue was immense for both sides, which is why Brooks's violent reaction can be seen not simply as a matter of personal vendetta but also as an extreme manifestation of the sectional controversy over slavery.