The comprehension of the governing mechanism which affects device instability is one of the most important requirements for the formation of reliable oxide-thin film transistors (TFTs). However, a quantitative analysis of the dominant mechanism of device instability, which stems from charge trapping induced by defects at the oxide semiconductor interface as well as in its bulk, has not yet been systematically performed. In this study, we examined subgap states, charge-transport dynamics, and various trap characteristics of oxide TFTs by multi-frequency C-V, pulse I-V, and transient current methods to achieve a comprehensive understanding of carrier transport and charge trapping mechanisms. We found that the charge trapping behavior of the tested amorphous InHfZnO (alpha-IHZO) TFT follows a multi-trapping mechanism, such as temperature-independent fast transient charge trapping by resonant drift of the injected electron and temperature-dependent slow transient charge trapping by charge transport from occupied to unoccupied traps. Understanding fast charging and slow charging described in this study can help to understand the root cause of device instability of oxide TFTs and ultimately improve stability and reliability characteristics.