The experimental conditions responsible for the drastic decrease of a power-threshold of parametric decay instabilities at auxiliary electron cyclotron resonance heating (ECRH) in the magnetic traps when the upper hybrid resonance (UH) for the pump wave is absent were analyzed. It was shown that in presence of a nonmonotonic (hollow) density profile, originating due to the 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 (waves) energy losses from the decay layer and the substantial increasing of the nonlinear pumping efficiency. The latter decreases the power threshold of nonlinear phenomena excitation which can be easily overcome in the current ECRH experiments utilizing the 1MW microwave beams. Different scenarios of the extraordinary and ordinary waves are investigated. The secondary decays of primary daughter waves and the 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 describing the anomalous phenomena discovered in the ECRH experiments at different toroidal thermonuclear devices all over the world.