The fire energy was clearly not at all stable within the body of the monster. The blood had been the very thing that forced the beast and tamer together. But whatever it was caused this unstable reaction.
There was also the fact that the fire type energy within the beast was mixing with the unstable blood. It was enhancing the energies that the dragon blood within the draconic flame salamander had brought to this mix and mash.
The tamer side had just added more emotions and enhanced the energy concentration by burning away the bond that had held the two in balance before. These two facts had brought the tantrum to a much worse state than what it had even intended.
The monster had just experienced pain. While the human side had felt pain, it had never felt pain like this. The beast had been spoiled and allowed to have whatever it wanted a sling as it was strong. That meant it also had no idea how to handle pain.