Board Thread:General Discussion/@comment-25489184-20150416125102/@comment-174.62.195.45-20150621183915

It's because the muscles he has from track are much denser than ones that are used for swimming, so therefore he sinks when trying to preform strokes that require light muscles. He is able to do butterfly because it requires a lot of upper body strength and he has the muscles for that particular stroke from track.