The planned execution of Shadrach, Meshach, and Abednego takes a sudden and deadly turn when the intense flames intended for the condemned end up consuming the executioners themselves. A combination of royal pressure, extreme heat, and divine providence seals the fate of the king's guards.
Driven by the king's fierce anger and his urgent, forceful commands, the furnace is heated to a severe extreme. The primary approach among commentators explains the death of the guards as a result of natural and psychological factors. The king's panic and insistence create a frantic atmosphere. Consequently, the strong guards who bind the condemned men act with reckless haste, abandoning the basic safety precautions required near such a massive fire. Had they not rushed so blindly, or had the furnace not been heated to such an unprecedented degree, they might have survived. However, the deadly combination of their carelessness and the overwhelming heat leads to disaster [מלבי״ם, מצודת דוד]. Typically, a furnace of this kind releases mostly smoke, but the extreme over-heating causes a massive eruption of sparks and fierce flames [ביאור שטיינזלץ], which directly strike and kill the guards [רש״י, אבן עזרא, מצודת ציון].
Beyond the natural explanation, other commentators view the death of the guards not as an accident, but as a deliberate punishment guided by God [יוסף אבן יחיא]. In this view, the guards actually overstep their duties. While the king orders the furnace to be heated, he has not yet given the final command to cast the men into the fire. Driven by their own cruelty and a fear that the king's anger might cool, prompting him to cancel the execution, the guards take it upon themselves to carry out the sentence immediately. This wicked, unauthorized rush makes them fully deserving of their fiery punishment [אלשיך].
The physical structure of the furnace and the timing of the events explain exactly how the condemned men end up in the fire. The main opening of the furnace is located at the top. The guards manage to carry Shadrach, Meshach, and Abednego up to the very edge of this opening. Just before the guards can push them in, the lethal sparks burst forth and kill the executioners. Left standing at the edge, fully bound and unable to balance themselves, the three men simply tumble into the furnace on their own [אלשיך, אבן עזרא].