The word has a new politcal meaning as well which I think is where a lot of modern usage comes from. From a political point of view, a cuck is someone who cheers or sits idly by with a smile while everything they should hold dear is fucked right in front of them.
If actual cuckoldry is becoming more common, and I suspect it is at least a little, it might be because people tend to fetishise the things they fear so that they can "own" them. Men probably have lower self-esteem now than ever before. I wouldn't be surprised if it's rooted in that.
Even if they "own" that kind of thing, I think it's probably still a pretty depressive circle. It's a way of making peace with low self-esteem rather than trying to fix it, like fat acceptance and things like that.