"It was a lift/return and unfortunately landed badly."

While this quote pretty much sums it up, the video would be a good way for an informed view.

Remember - "slam" is really a misnomer...there is no "slamming" required. The rule says you have to bring your opponent to the mat without unnecessary force. The case book has an example where you accidentally drop your opponent and that's a "slam" because you have to bring your opponent to the mat under control and protect him. Its just an unnecessary roughness call but because we call it a "slam" it sounds like it is supposed be a big whack or something really out of the ordinary. It probably doesn't get called enough. Unfortunately, when it ends in an injury it costs the offender the match. There's rarely a "bad motive" and usually it is just what happened here...the lift ends badly...for whatever reason. And most of the time the injured wrestler is able to continue so its just a point.