Determining the mechanism of transition from projectile-impact ignition to detonation is a complex and difficult task with strong practical applications. Ignition due to low-velocity projectile impact cannot be properly explained by the available theories. We attempted to determine the mechanisms of initiation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)-based polymer-bonded explosives (PBXs) in a range of high temperatures, which have rarely been investigated. Comparing the shock initiation results, we found that the low-velocity projectile impact response mechanisms for a heated explosive are much more complex. Our results show that the impact ignition threshold velocity of the heated explosive does not always decrease with increasing temperature as commonly expected. A temperature dependent plastic power during impact controls the ignition in the range of 25 degrees C to 75 degrees C. At 190 degrees C and 200 degrees C, there was a sharp rise of reaction degree induced by phase transition for high HMX-content PBX. Conversely, such phase transition effect becomes insignificant for low (<50%) HMX-content PBX. Our results show that three competing mechanisms affect the impact safety for a high HMX-content PBX at high temperature, including plastic power, temperature sensitizing, and phase transition.