The River did it! The Missouri River, that is.
This river altered the Kansas almost perfect rectangle shape, by carving its signature in the northeast corner. The Missouri streams from the northwest, cuts out the Nebraska eastern border, and proceeds south to carve part of the Kansas northeast border before it continues into its name State.
Here it streams into Kansas City, starting on the Kansas side of the city — KCK, and continuing westward to the Missouri side, KC-Mo.