Consider this example, someone creates a very simple plugin for FF that all it does it inserts a meta-refresh tag for a page the browser downloads. Now this meta tag will trigger a periodic refresh of the page and will be recorded as a human action. The browser could allow jscripts so it will pass through the GA filters but in reality this represents automation or what I could call a bot. Of course there are lots of other implementations but you get the idea.
Motives for it? Lots, say for instance you have a marketing campaign in place like ppc.