While there are a myriad of factors, I lay the blame with the parents of those parents, at least for the people I know personally who have done this. I am 27, and will readily admit that the majority of my generation and those younger are raised with an overbearing entitlement mentality. People who are spoiled and self-centered don't have the room in their hearts for children. This may sound harsh, and certainly doesn't apply to all of them, but I do believe that's why there's a rising trend. Even at my age, I've already offered to take care of three different children within the extended family for my cousins who didn't want them. Two of those kids are now with their grandparents -- one still with her mother. That's in addition to my own three. I hate to be an enabler, but concern over the children's welfare won out.
If it were one of my kids who didn't want their kid, I'd offer to help teach them more hands-on parenting while they still do it themselves. Some of these people have told me that "I'm just not a good parent, the kid will be better off with my parents." They apparently don't realize that ALL parents feel that they're failing at some point, but they stick with it and learn as the kids grow. It's a sad trend, and one that I'd hope will eventually reverse for the sake of the newest generation -- character qualities in general are already on the decline.