The suppression of the current collapse phenomenon is required for reducing on-resistance in AlGaN/GaN high-electron-mobility transistors. Current collapse is caused by electron trapping at the surface and in the buffer layer. In previous works, electron injection to traps has been mainly regarded as being due to gate leakage current; however, other factors have not been discussed. In this study, a novel current collapse mode induced by source leakage current is examined with a 2D device simulator. In addition to gate leakage current, electron trapping is induced by source leakage current that flows under the gate electrode during the off-state. The electron trapping increases on-resistance and decreases pinch-off voltage. The increase in on-resistance is clarified under several trap and voltage conditions. In the simulated range, it is revealed that the on-resistance increases over 1.31 times by the source current collapse mode only, when the stress drain voltage of 500 V is applied during the off-state.