So we know that right at the start, Terra is rescued from the Slave Crown by Arvis and escapes into the Narshe mines. Here, she's cornered by guards and a section of cave gives way, dropping her to the floor below. Here, she waits unconscious until Locke saves the day. Then they run off to Figaro and it's all a case of trying to get Terra to help the Returners.

What if that section of cave didn't give way? Terra would likely have been captured. Would Locke have made it in time, or would he have had to fight off the guards and rescue her? Would he sneak into Figaro and ask Edgar for help? WOULD HE DO NOTHING?

It's just very convenient that a natural "disaster" for want of a better word is the only reason things could progress as they did.