Development is the main culprit with EC. When you look at other historical floods for them they are mostly river based and not from water flushing down the hills and streets. Very little is required of developers for making sure that the down stream infrastructure is adequate for the size of neighborhoods they want to build. So you have land changing from permeable to needing SWM. The thing about that is the velocities of the run off has changed drastically. A new development will get that water off its area and into the natural drainage a hell of a lot faster than if it were trees and soil. Once you get a ton of water moving at a fast speed it creates its own path.
Oddly enough, a storm drain 85% full carries more water than a pipe 100% full.