I'm guessing he slept w/ someone while he was out of town...
But, that's just me and my paranoid nature.
As for Dig's advice? Show me the hottest woman or man in the world, and I guarantee you that I can find someone that is sick of that person's shit.
The basic crux here is that you may not ever have a clear and honest answer from him about what went wrong, and why. Obviously one wants to be angry and call the other person names (see above) for hurting you, but the truth is he may very well have done you a favor and cut you free to find and develop a better life for yourself.
He pretty much owes you no explanation. He's not required to give you one. Even if you do get one, it will probably be sugar coated enough to sound hollow. It's not you, it's me. Which, is actually quite often true. Maybe he feels you are a good person, but not a good match for him as a wife. He's just done a poor job of communicating why he feels this is true, or it may be an intangible feeling he has based on a general fear of commitment.
In the end, does it really matter? Are you going to let his opinion of why things didn't work color your self esteem and self worth?
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."