The part of the "why" that the two of you need to figure out is the part that will ensure that she does not do it again. That is where the Marriage Builders principals come in: Radical Honesty, Policy of Joint Agreement, Identifying and meeting Emotional Needs, eliminating private behavior, setting boundaries, etc,etc. It sounds like you are well on your way to working on all of those aspects of recovery.
The thing that I still had a hard time with was the fact that HE DID THIS TO ME. All of the work that we did relating to Marriage Building did not eliminate the fact that HE DID THIS TO ME. I would ask him questions and talk to him in some way I think hoping or wanting to find that HE DID NOT DO THIS TO ME, or the answer to why HE DID THIS TO ME. Sure, I found some answers. But the real answer I wanted was not available--the answer that said:
'oh, no, nothing like that really happened, how could something like THAT happen to US, no it did not happen.'
You know that answer that you are looking for?....You won't find that answer....Because it did happen. Sounds like you caught it in its early stage and she got a grip on herself.
Are you setting limits on the time that you spend grilling

her to find out why she did it to you and to try to find out that she did not really do this to you after all

? Seriously, it is good to set a time frame for these questions and provide some limit to that time frame. Keep date night if possible for fun and re-connecting to each other. If you are like me, it was hard to do that, but the two of you will be better off if you can enjoy each other on some nights with no affair-talk.