Generally that happens because a node contains a long string of text without any character pairs the browser feels it can insert a line break between. This is considered by some to be anti-social behaviour. Feel free to down vote such nodes. ;)

Well, while I feel annoyed too by this, when I stumble into it, and although I do see that smiley... most authors that cause this to happen do so because they just don't know and use <pre> tags, which is not really an excuse for not reading that they're discouraged anyway, and that one should really use <code> tags, which in turn is clearly indicated just under the preview button in the first place. However a much better recommendation would be to /msg the author to the effect of letting him know, or reply to his post, and in the course of the reply also letting him know, or to /msg the janitors for some intervention to be done.