My best educated guess for the reason most SPS encrust before going vertical is to stake out as much territory as possible. Once they bump into another coral or chemically detect that other coral they will start to branch. When a SPS encrusts they are using the rock as their skeleton, but when they branch they have to build their own. Also if you think about building a solar panel array, do you build it with one panel above the other panel or do you spread them out on a flat surface, which is more efficient for capturing energy. Why build a skyscraper when you can build a ranch. Again just an educated guess, no experiments or articles to back up my train of thought.