This paper investigates the origin of the threshold switching effect in NbO₂. It is found that the effect is independent of the metal-insulator-transition but can be explained by a trap-assisted Frenkel–Poole like conduction mechanism in combination with a moderate temperature increase by only 150 K due to Joule heating. These findings lead to the development of a physics based model which is of pure electrical nature and explains the occurrence of the threshold effect as well as the negative-differential resistance behavior observed in NbO₂.