I don't think you are doing anything wrong. I believe it just happens when either large pictures are posted on to the thread, or some annoying git types in a word/words with no spaces that is longer than the default width of the page.
Example..............................thisisthesortofthingIam talkingabout...........!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!