GaAs thermal smoothing at temperatures T ≤ 650°C in the conditions close to equilibrium yields surfaces with atomically smooth terraces separated by steps of monatomic height. At higher temperatures T ≥ 700°C, surface smoothing is changed to roughening. In the present paper, possible reasons of surface roughening at elevated temperatures are studied by means of Monte Carlo simulation and compared with the experimental results on GaAs. It is proved that the thermodynamic roughening transition, which consists in spontaneous generation of atomic steps due to decrease in the step line tension down to zero, cannot explain the experiment because it should occur at temperatures T ∼ 1800 - 2000°C, i.e. much higher than in the experiment. Kinetic instabilities caused by deviations from equilibrium towards growth or sublimation are shown to cause GaAs roughening at elevated temperatures. The microscopic mechanisms of kinetic-driven roughening are discussed.