What is one reason that white mobs attacked blacks in northern cities in the 1800s?

The articles of confederation: article ii. each state retains its sovereignty =, freedom and independence, and every power, jurisdiction and right, which is not by this confederation expressly delegated to the united states, in congress assembled article iii. the said states hereby severally enter into a firm league of friendship with each other, for their common defence, the security of their liberties, and their mutual and general welfare, binding themselves to assist each other against all forced offered to, or attacks made upon them, or nay of them, on account of religion, sovereignty, trade or nay other pretence the united states, in congress assembled, shall never engage in a war.. nor enter into any treaties or alliances, nor coin money.. nor borrow money on the credit of the united states, nor appropriate money, nor agree upon the number of vessels of war to be built or purchased, or the number of land or sea forces to be raised, nor appoint a commander in chief of the army or navy, unless nine states assent approve to the same, nor shall a question on any other point, excerpt for adjourning from day to day, be determined, unless by the votes of a majority of the united states in congress assembled summary