In this letter, the underlying physics of threshold voltage (Vth) instability and the eventual device failure mechanism of 100V Schottky p-GaN gate high electron mobility transistors (HEMTs) under repetitive short-circuit (SC) stress with varied drain voltage (VDD= 40-70V) and SC pulse duration (TSC=10 μs & 20 μs) is studied. In the lenient SC stress with lower SC energy (e.g. SC stress @ VGS=6 V, VDD=40-70 V, TSC=10 μs), the devices exhibit significantly positive Vth shift while the Vth instability shows positive dependence with the stressed drain voltage and the repetitive SC pulses. For device stressed at VDD=70 V with 150 SC pulses, a substantial ΔVth as high as +0.68 V is observed. Such a prominent Vth instability is induced by the electron trapping in the p-GaN gate region during the SC events, which also results in the suppressed gate and drain leakage current after SC stress. In the more stringent SC stress (VGS=6 V, VDD=70 V, TSC=20 μs) with much higher corresponding SC energy, the device failed due to the drain electrode burned out initiated by the significantly high SC energy during the SC events.