The item description for the shield Sura's Supper Plate states that it retaliates when hit by a melee attack.
The paladin ability Reinforcing Exhortation is neither melee nor an attack. After using it on Eder who is currently wielding Sura's Supper Plate Pallegina apparently gets hit by this retaliation effect.
Also worth noting is that it might only happen after using the fighter ability Vigorous Defense first (tested once).