Well when we did the art/animation/implementation for Illidan about 3ish years ago, we wanted Illidan to do a cool flip animation over the target. In order to do that, we had to create time for him to be able to flip over the target.

Well, flipping over a fast-moving target doesn't really work visually, so we decided to apply a very very short duration slow to illlidan's target when he dives.

This was all before he went through his first (of what would be many) tuning passes, so it was a large consideration of what we call his "power pie", and he is tuned around having it.