Question

why tendency to form dπ-pπ multiple bond decrease down the group?

Answer 1

It decreases in order of increasing in principal quantum number. The bigger value corresponds to longer and wider orbitals, but each of them is still occupied by two electrons. Therefore, upon formation of dπ-pπ multiple bonds, the efficiency of overlapping is much smaller and energy of such bond is found very small. Thus, dπ-pπ interactions becomes very weak for the orbitals with big principal number.