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: There are simple and effective solutions in use today that guarantee a reactor shutdown if coolant power is lost. Simply suspending the control rods above their slots via electromagnets is probably the simplest. If the magnets lose power the control rods drop into the reactor and bring the nuclear reactions to a stop (or near enough anyway).