Enemies run away

Been playing Skyrim for a long time, but never made it to level 50. (Shame).

I'm getting up there now and I noticed sometimes as I'm fighting melee, which isn't often, some enemies will run off. I don't believe I have any magical or enchanted gear that would cause that and I'm not a vamp or anything, just high leveled sneak archer (how original).

Does anyone know why they would do that? Maybe just don't want to die?