If you look at that particular article in the html mode (there is a button in your editor) you will see a lot of code has been inserted in it. Remove all the code you do not need <div> tags or <span> tags. We see this quite often especially if people cut and paste from another source (M$ Word is the worst culprit) as the cut and paste copies all the layout code from an article too.
It probably took a while for the problem to show because of the cache in Joomla. Sometime you can add an article but the cached version is displayed for a while and only then would the problem manifest itself.
