Sorry for the late response. I'm using Google Chrome. JMJimmy's onto something with the addon theory. I don't get this behaviour in IE, and when I disable all my add-ons in Chrome, it also doesn't happen.
It looks like it may have been one of the form auto-fill settings in Last Pass. Disabling that plugin stops it from happening, re-enabling it causes the problem to return.
Nothing to see here, move along