Bas Rutten does seem to have a point in the video. In Silva's situation that is not the case. His leg was facing into his opponent' s leg. Look at the video.

My guess for the break is that he stuck his opponent near the knee. Where his opponent's leg will be thicker and denser because of muscle and tendon attachments. Your bones are denser where there are muscle attachments. Silva also struck with a part of his leg where there is probably less density because there aren't muscle attachments there.