That's the case for any nuclear meltdown, Chernobyl isn't special in that regard. If you have a very dense blob of liquid fuel (called corium after a meltdown) generating megawatts of power it's going to melt through concrete and steel, it's just a matter of time. Once that blob hits a ton of water underneath it forcing all of that water to heat up will create enormous pressure and the only way out is up.