Originally posted by Gunther Ultrabolt Novacrunch:

I had a screenshot of the floating node. I had it completely separated from all rock/ground. I'll do another for science.

My theory isn't that is has to be disconnected from all surrounding ground AND there can be absolutely no left over ground on the node itself.
This might explain why it won't explode right away but sometime while you are mining the node. You hit that little chunk of ground and then it explodes.