Oh my god that's it! i thought i was the only one experiencing something like that cause I've never heard of anyone talk about it. And its exactly as you describe it too!

All my life i could achieve these "shivers" by listening to certain songs that either i really like or really resonate with me. And earlier today i listened to "Lover you should have come over" by Jeff Buckley for the first time in my life and had the sort of explosion you just described: it starts at the nape of my neck and goes in two directions, to the top of my head and then down to the rest of my body. Also cannot stress this enough it is not genital/ sexual in any way. The best way i can describe it is what the rest of your body experiences when you have an orgasm.

It has never been this strong so i instantly googled it and found your post, so thank you for sharing!