A schematic of the set up considered in the text. A giant planet with mass m₂ follows a circular orbit with semimajor axis a₁. Exterior, lies a lower-mass planet m₂ on circular orbit with semimajor axis a₂. The exterior orbit is forced to undergo nodal regression due to a combination of stellar quadrupolar potential and secular perturbations from the inner giant planet’s orbit. The giant’s orbit, in turn, is regressing mostly owing to secular perturbations form the stellar oblateness. Initially, the inner planet regresses faster but as the stellar quadrupole decays (as a result of physical contraction), a commensurability is encountered in the two frequencies, leading to the secular resonant excitation of mutual inclinations between the planets.