My bad - this is a browser-specific issue that was happening in Firefox but not IE.
Here are the css styles to resolve this in other browsers (I have only tested that this works in IE and Firefox).
white-space: pre-wrap; /* css-3 */ word-wrap: break-word; /* IE 5.5+ */ white-space: -moz-pre-wrap; /* Mozilla */ white-space: -pre-wrap; /* Opera */ white-space: -o-pre-wrap; /* Opera 7 */
That usually happens if you have words that itself are longer than tooltip width. Setting the proper width should resolve the issue.