I was really upset once about something my in law said to me and I kept it in for the longest time because I don't want my partner to get involved in the matter as it is after all his mother. A week of holding it in and I just broke down and he finally found out. He cried as much as I did because he felt guilty that his mother said those hurtful things to me and also because I was afraid to tell him.
Perhaps when they are discussing some personal issues? Or discussing something that would affect them both and both feel helpless about the situation in front of them? Or it could simply be that it pained him to see his wife crying..
Usually something very personal or when his wife really managed to do something to move him. Lost of something or someone precious could trigger this as well. Well, ultimately both r humans.
Divorce? Death? Anything that leaves the feeling of hopelessness or helplessness behind.