I think it's a lot to do with the sheer numbers. Then there's some people who just like to disagree with other people. Most of the ones I read don't generally get too bad. If someone posts a snide or negative remark, others will generally take them to task for it and often the OP apologizes. It's hard to judge tone with just words, and rav's interface doesn't allow for emoticons, which can help.
sue- knitting heretic