I immagine it might be a combination of factors that cause some MLCers to get through this stage in life without blowing up the family.
From the couple of cases I know in RL, I would say that the original triggers were the same: parent/peer dying/oldest kid going away to school, the anger/grumpy/always complaining man/woman is the same, but possibly there needs to be a second major trigger to push them into a full blown crisis as opposed to a transition. The appearance of an OW could do this, for example. Or maybe even extreme financial difficulties: house going into foreclosure, business failing, gambling, huge debt. Like a pressure overbuild.
I would also say how the LBS is behaving at home at that point might also be a contributing factor. Many on here are completely shocked on the day they are bombdropped, with their Hs having just told them they loved them that morning, but others, like me, had been going through this nightmare for a few years already without knowing what it was. Our Hs had distanced themselves, were extremely difficult to live with, did not pull their weight and if you were like me, you were complaining to them a lot. This might also have been a contributing factor. Before you all start getting heated up, I'm not saying it's a fault of the LBS, just that she unknowingly added to that pressure.
I know from friends and family whose kids have had drug problems in the past, that it takes a combination of 'perfect' situations to lead a kid into addiction. Take away one of those major contributory situations and the addiction can't take off. Almost like the triangle of fire if you know about that: fuel, oxygen, spark. Without any of these 3, a fire can't happen. I think it's similar with MLC as opposed to MLT.