This is funny to me because many people here (west coast of Scotland) would be more likely to identify as Protestant than think of themselves as Presbyterian or even Christian, for obvious, uh, sectarian reasons.
The trend in the US must be very much because Catholicism became destigmatised in wider American society and as WASPs lost cultural hegemony.
That's been true for a very long time. Like I said note that the intersection point is around age 55.
Also as I noted above some of these "Christian" identifiers probably were raised Catholic and have never been formally Protestant. I believe there's even a Democratic member of Congress this applies to, as in she (I'm pretty sure it was a woman) put down her religion as "Christian" in that Pew survey but said in an interview she was raised Catholic but no longer identifies as such...but I can't remember who or if I'm totally just thinking of someone else who I confused with a Democratic member of the House.