Not that you need to hear this, but if they can't or want give you an explanation, more times than not there is a third party involvement somewhere. Prayers to you.
Yeah, " just not happy" at home almost always means "i think i would be happier over there". Which is a lie of the devil. Marriage takes work, for better or worse. And commitment for a lifetime. Im afraid alot of people just forget exactly what it was they signed up for, or they never really thought hard about it to begin with. A marriage is a covenant before God between two people, a man ansd a woman, that joins them together as "one flesh". Thats pretty serious business. Ill spend the rest of my life working on and for my marriage. Thats the deal i made.