What is a switching node island?
Read a bit more about ground bounce. It's basically the area with your switching supply.

http://www.analog.com/library/analogdialogue/archives/41-06/ground_bounce.html

Basically, a switching supply is a noise machine and a solid ground plane can be a bad idea underneath them. I will let you read more from the Analog website because if I could describe it better than them, I'd be working for them.