Amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) have attracted much attention owing to their promising applications, such as display devices and dynamic random access memory (DRAM) devices. This study reports a comprehensive study on the bias temperature instability of a-IGZO TFTs. We analyzed the behavior of transfer characteristics, under repeated bias stress and recovery at different temperatures, to unveil the degradation mechanism of bias and thermal stress. Based on threshold voltage, subthreshold swing, and field-effect mobility, the correlation between transfer characteristics and stress time was found to depend on temperature, revealing the opposite trends. The trends are interpreted using two mechanisms: trapped electron and oxygen vacancy. The consequential behavior of transfer characteristics under repeated stress and recovery depended on the competition between the two mechanisms, and the result of the competition depended on the temperature. The predominant mechanism was determined based on the temperature, and a high temperature enhanced the generation of oxygen vacancies, resulting in a negative shift at high temperatures. Repeated stress revealed that the predominant mechanisms were maintained with constant V-T shifts over cycles, and repeated recovery confirmed the difference in recovery mechanisms with increasing V-T shifts over cycles. Understanding the competition between the two main mechanisms aids in explaining the bias temperature instability of a-IGZO TFTs.