[shkkmo]

The xenon poisoning is why they took the control rods all the way out, but the runaway reaction was probably caused by the graphite tips on the ends of the control rods. As the control rods were scrammed these tips passed through areas of high neuton flux and caused a spike in power which probably caused the explosion.

There were a lot more factors at play in the chain, but the burnoff of the xenon wasn't itself the proximal cause of the explosion.