I think it is due to the d-orbitals in Cr being the first ones (3d) they don't have to be orthogonal to inner shells and are therefore relatively more strongly bound than 4d or 5d orbitals in the higher periods. Similar trends can be observed in the other groups, too, e.g. V vs. Nb or Ta, Fe vs Ru or Os.
Ok, could you place here some links... because it's better if I read something for first. The only phenomenon I'm aware of is (ns) orbital energy fall due to Pauli repulsion, and obviously this isn't the entirely same thing.