Electron orbitals are most stable when they are fully filled or half – filled. In the case of Chromium, and Copper after the 4s², 3d⁴ and 4s², 3d⁹ respectively configuration is attained, an electron from the 4s orbital jumps to 3d subshell because 3d⁵ is a much more stable configuration than 3d⁴ and 3d⁹. That's why chromium and copper exhibits such electronic configuration.