Chernobyl was a graphite fire accident with nuclear material involved in the burning graphite. Combining nuclear fuel with burnable graphite seems like a bad idea to me. But I am not a nuclear engineer...
Chernobyl was also water cooled. Steam generating cooling pipes going through the graphite moderator. Now I not sure about the graphite form of carbon, but if you pass steam over at least amorphous carbon (such as coke) at an elevated temperature, you get carbon monoxide and hydrogen. Carbon monoxide besides being a toxic pollutant is a low BTU fuel gas. Concentration reaches the LEL (lower explosive limit), all you need is one little spark or other heat source and BOOM (this also applies to methane, propane, any inflammable vapor).
Add to that Chernobly never really had any kind of pressure rated containment.