How does a global flood decide to carve out a massive canyon in one particular spot underneath the floodwaters?
The global flood didn't decide it, God did. God decide to put that mass there, as He knew a flood would be necessary. Once it is realized that God decides what goes where, the rest is easy. You learn not to fret over much of anything...
More to your point, there's such a thing as wind and water forces and different strengths involved with the layers. A fraction of what occurred with wind and water forces, showed itself with Katrina and any other wind or water disaster you'd like to apply here...