I'll speak on my wife's behalf and say that her libido increase had to do with 2 main things: 1. her age (shes 44 now) 2. and her bf. I'll preface by adding that she was never a sex nympho back when we were still monogamous. We would have sex about 2-3 times a week. She was very content and even thought 2-3 times was too much at times. That was in her mid to late thirties after having 2 children.
But her libido gradually started to increase the last couple of years when she entered her 40s. Unfortunately, during her libido climb, I was on the verge of getting into my libido decrease. When she finally brought her bf into her sex life, they would have sex 2-3 times a week just like we used to at the peak of our libido as a couple. But that wasn't the thing that surprised me the most about my wife's libido increase. What surprised me was that her sexual limits increased as well. Her "size limit" increased and her interest in kinky and more exciting sex also increased. If you had mentioned kinky or exhibitionist sex to my wife a couple of years ago, she would have scoffed. It's a completely different story now.