I can briefly describe what *might* have been a causative factor in one of the marriages.
Husband and wife married for many, many years. Husband has some undiagnosed (minor) neurological problem (don't want to go into too much detail). It keeps getting misdiagnosed and throws him into a severe depression. He is so depressed, he finally ups and leaves for 2 months on a walkabout in the woods. Of course this stresses the wife out, understandably, and she has to take sole responsibility for everything (kids, house). He comes back 2 months later, gets proper treatment, and everything is fine for him. But not the wife. She now wants control of everything and second guesses everything he does around the house. They are in counseling but somehow she cannot really "trust" him again to be the head of the household. This goes on for years. It erodes their relationship, yet they still enjoy their "home", their kids, their life and all the trappings. There are deep, established family ties at this point. Yet, he meets another woman, is friends with her for many years, and then starts a "relationship" which continues for many years.
So, I sort of think this example was the catalyst for that marriage. Not living in the house, I'm not sure I can understand all the dynamics; however, I do know that the whole depression/rejection thing is probably very crushing for a man's ego (or a woman's for that matter). Connecting with someone else was proably inevitable.
The second marriage I know of was purely caused by marrying too young, getting deep in debt on both sides and not being able to get out (they actually tried).