This is something that happens often, perhaps even more so now thanks to cuck porn. There's usually a trigger (discovery of hubby's affair, he's a fat slob, etc.) for this behavior. Once they start they can't stop and often are more sexually aggressive than younger women. Middle aged women of today are, in many cases, much more fit than they used to be. It's not difficult to find beautiful middle-aged women (especially if their rich husbands forked over for some plastic surgery). Women at this age are more confident of themselves than when they were younger. Men can sense this and act accordingly. To your last point I don't think many husbands know. I've heard second hand numerous testimonies to this fact.