I don't know why for her, and I think it could be a combination.
I have my opinion for why your wife did it:
1 She was relatively happy but routine, not "butterfly in the stomach" excited.
2 She took you for granted that you would never leave her (she probably knows you better than you know yourself)
3 Plus, she didn',t think you would catch her, so you would never know
4 The other times she broke boundaries, flirted, went too close with a male, even cheated, either you didn't know or you did very little about it
5 The other man was marginally physically attractive but he was bold, arrogant (to her, confident, to you, an arrogant ass), aggressive, powerful (her "superior"), he made her feel DESIRED and FEMININE, intelligent and skillful (more than her) so she could respect that from him, SUCCESSFUL at work and financially (he has a beach house, perceptions) possibly, likely, other female workers have expressed they found him attractive, she felt like the "prize" he chose her - add as many of these as you want - my experience, power is an aphrodisiac (as a teenager as an employee at a fast food restaurant, female workers swooned at the assistsnt branch manager, and I've seen it many times since then with my own eyes with the "boss"), definitely two things he had was humor and confidence (likely arrogance, but that's another story - you can't be too cocky sometimes and this looks like one of those times)
6 She transferred her allegiance from you to him at some point.
7 Your lack of psychic abilities hurt you in her eyes, here she was obviously falling in love and you couldn't even see the obvious signs (she probably gave you some lack of loving words or affection actions and you didn't notice it, not really, "obvious" except in her mind) and she has some immature idea that her "soulmate" would have noticed this, so you must really not be the true soulmate, maybe the other man is, maybe it is "fate" and "destiny" (this is from my youth of having a sister's bedroom next to mine and me hearing such horseshit regularly, plus having another adult sister getting married and being an adult in every possible way - smart, reliable, mature - EXCEPT for matters of the heart, her "love" mindset was set back at age 13-14-15 mentality - as were so, so many friends of theirs).
I don't know why she would risk it really, I can only guess and give you some ideas, you are in the position to see which shit flies, if any.