cancel is not a valid value for a type attribute, so the button is probably defaulting to submit and continuing to submit the form. You probably mean type="button".
(The javascript: should be removed though, while it doesn’t do any harm, it is an entirely useless label)
You don’t have any button-like functionality though, so would be better off with:
<a href="http://stackoverflow.com"> Cancel </a>
… possibly with some CSS to make it look like a button.