We have stressed GaN High Electron Mobility Transistors (HEMTs) with AlGaN back barrier on SiC substrate at high voltages at normal and high temperatures. A pattern of device degradation differs from what occurred in reported GaN-on-SiC HEMTs and GaN-on-Si HEMTs. The recoverable degradation of drain current is observed under Vds=0 step stress condition, but no degradation is observed under off-state condition. We attribute the results to electron injection into the back quantum well, other than electron trapping. Raman scattering indicates planar strain is greatly enhanced under Vds=0V condition, which is likely to assist electron in tunneling the back barrier.