Putting yourself in her shoes, (my situation too), I can see how the stress and guilt of keeping this up could cause the hostility and emotions.
Think of how emotional and wrecked we have been. Now think of what she could be feeling to have "caused" that and still feeding that fire. Now add the fact that they are dealing with two "relationships" that are chaotic. Seems overwhelming to me.
She may have the new car scent with the OM, but that cannot cover up the smell of the body in the trunk. (sorry - excuse my poor excuse for a "funny?" analogy)
My W told me several weeks ago that she felt that she has lost herself because of all of this. She actually felt like she had no power (even though I was thinking she had all of the power at that point).
She was being pulled by me and the OM. She wants out of both at times. If I knew what I know now, I would have just validated what she was saying. But I probably added to her stress.
Me-45 W-44 S21, S18, D15 T-27, M-21 BD Jan 2014 PA revealed March 2014 In-house separation - April 2015 I filed - Aug 2015 She moved out Oct 2015