Coal can be very slow burning and I'd guess it doesn't need as much oxygen as, say, wood to have a sustained burn going. I'd imagine it's kind of like a peat fire. Once that starts to get underground there's less oxygen exposed to it, so it smoulders rather then straight up burns. I can last for as long as there is fuel to burn in this state since there is still oxygen present, but not enough for the fire to burn itself out quickly.
I'm guessing with this a lot of oxygen was originally present due to the mine shafts and such, which let the fire burn hot enough to open the cracks and fissures. Once that got going there's enough oxygen flowing through the area to allow a smoldering fire. Look at the damage and think about how long this has been aparently going on for. This is a very very slow burning fire.
