In the experiments with combined ECRH and NBI plasma heating started at the GDT facility (Budker Institute) in 2014, two quite different scenarios of plasma discharges were realized. The first one was characterized by a broad radial distribution of the absorbed ECRH power and a stable plasma with improved energy confinement of fast ions, while in the other the electron temperature profile was highly peaked with local values of Te > 500 eV, but plasma was susceptible to low-frequency instabilities that had a net negative effect on confinement. We discuss the first theoretical explanation of these phenomena. Essentially different temperature profiles are considered as a result of self-consistent evolution in time after the ECRH is switched on. To explore this concept, we combine an advanced quasi-optical model for the calculation of microwave power absorption with a simple transport model describing electron energy balance in a gas-dynamic trap.