This paper investigates anomalous capacitance-voltage (C-V) degradation in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film-transistors (TFTs) under hot carrier stress. In vacuum hot carrier stress, both the gate-to-drain capacitance (C GD) and the gate-to-source capacitance (C GS) curves exhibited positive shifts due to electron trapping in the gate dielectric. In addition, an observed increase in capacitance value at a lower gate voltage in the C GD measurement only can be ascribed to interface state creation. However, when the hot carrier stress was performed in an oxygen-rich environment, the C GD-V G curve showed a significantly positive shift due to the electric-field-induced oxygen adsorption near the drain terminal. The degradation in the C GS-V G curve is due not only to the positive shift, but also the anomalous two step turn-on behavior. This phenomenon can be ascribed to the electron trapping in the gate dielectric and electric-field-induced oxygen adsorption on the channel layer, especially in the area adjacent to the drain terminal. The electron trapping increased the source energy barrier, with the electric-field-induced oxygen adsorption further raising the energy-band near the drain, resulting in a two-step turn-on behavior in the C GS-V G curve.