The experimental conditions responsible for the drastic decrease in the power-threshold of parametric decay instabilities under auxiliary electron cyclotron resonance heating (ECRH) in toroidal magnetic fusion devices when the upper hybrid (UH) resonance for the pump wave is absent are analyzed. It is shown that, in the presence of a nonmonotonic (hollow) density profile, originating due to plasma equilibrium in the magnetic islands or anomalous particle fluxes from the ECR layer, and a finite-width pump, the 3D localization of one or both daughter waves is possible. This localization leads to the full suppression of daughter wave energy losses from the decay layer and a substantial increase in the nonlinear pumping efficiency. The latter decreases the power threshold of nonlinear phenomenon excitation, which can be easily overcome in current ECRH experiments utilizing 1-MW microwave beams. Different scenarios of extraordinary and ordinary wave decays are investigated. The secondary decays of primary daughter waves and pump wave depletion are considered as the most effective mechanisms leading to the transition of primary instability to the regime of saturation. The proposed theoretical model was shown to be able to describe the anomalous phenomena discovered in ECRH experiments in different toroidal fusion devices all over the world.