It would be very difficult to describe on paper what is looks like in the eyes of an official. I think the easiest way to avoid a call like this is to not bring someone down with force in the situation you described. In this situation what I saw was the defenders legs flailing, and then I saw his head hit the mat very squarely, accompanied by a very deadening thud. These are all fairly tale tale signs of an uncontrolled decent. As I indicated earlier I did not think that what happened was done on purpose or to harm anyone, but because of what I did see I had to make the call.
You know one of the things that litteraly scares me as an official is when a wrestler hits a double leg and picks their opponent up off the mat, and I've got to be in fairly close and I will always verbal warn that wrestler to "Bring him/her down easy" And the thing thats running through my mind is "Gosh I hope they don't just slam them down on the mat" I've seen it happen, I've seen concusions result from it, its very scary. Thats why at this level of wrestling im gonna error on the side of caution and expect wrestlers to in fact bring them down gently"
Obviously I don't mean lay them down on the mat liek you would a baby in a crib. But someone's head bouncing off the mat probably isn't a good thing.