Recently I upgraded my leadership skill and I've noticed in that time a sudden increase in the proportion of mining collapses which are costly in the long run. I've had no less than 10 collapses in the last month alone compared to maybe once a month (for a whole year) prior to my leadership upgrade.
So question is, what the hell is triggering these several sudden collapses and what is the formula used to determine such? I've gone through the mining rules page and it doesn't list any specific formula or chances of collapses.
The only factor that I have heard is the number of NPCs used in the extraction - the more NPCs, the higher the chance of collapse.
Using 0 NPCs will not protect you, though. I have heard of very costly collapses with 0 NPCs involved.
Let me add that I wonder why such a costly aspect of a primary sector of the economy is not covered by the rules.