A simple theory of spontaneous spin reorientation in rare-earth orthoferrites RFeO3 and orthochromites RCrO3 induced by 4f-3d interaction is presented. It is shown that both the temperature and the nature of the spin reorientation are the result of the competition between the second- and fourth-order spin anisotropy of the 3d sublattice, the crystal field for 4f-ions, and also the 4f-3d interaction. A case of the substitution of the rare-earth ions with a non-magnetic impurity is also considered.