It is called the "false grip". It is usually used to take the stress off of the shoulders for people with shoulder injuries, putting to use more of the triceps and forearms to lift the weight -- so many use it out of necessity and not preference.
There have been a lot of accidents lately using this grip to the point Dave Tate, one of the first people to use this grip, made a video about how to properly do it following the accident of that football player at some college where the weight slipped out.