Two answers to this:
1) Anonymity: any forum that provides the ability for posters to be anonymous is going to be ugly. Whether its running, gardening, or politics: anonymity breeds negativity.
2) It's not just LRC. Within any group that is hyper-focused on one thing (i.e., running, baseball, music, football, cooking, etc.) you will find people who are really invested in that particular thing. People who are invested to the point that they are going to spend their time on a niche forum discussing that particular thing are going to have strong opinions about the content. They are also more likely to be relatively gifted at that particular thing, which breeds a machismo that brings out the worst in them that is helped along by point #1 above.