Because they lost power to the water cooling system, they needed to vent the pressure that's building up inside.My suspicion is that as the temperature inside the reactor was rising, some of the metal cans that surround the fuel may have burst and at high temperature, that fuel cladding can react with water to produce zirconium oxide and hydrogen.That hydrogen then will be part of the gases that need to be vented. That hydrogen then mixes with the surrounding air. Hydrogen and oxygen can then recombine explosively. So it seems while the explosion wasn't directly connected with the nuclear processes, it was indirectly connected, because the hydrogen was only present because of what was going on in the reactor core.
quote: This reactor is NOT still running. The control rods did fully insert. But - since it was running at full power and had been for weeks or months, there is a lot of residual heat from nuclear decay that remains for hours and days. That is the heat that must be removed to avoid melting the fuel pellets.