frankenbeans
To be sure that an app has fully stopped working make sure you press "force stop" in its settings page. Other wise the app can keep running in background.
This usually should not be the case for browsers, but in this case it seems it was running. This might be because of a particular page you were on.
Its possible the app crashed but didnt fully terminate, and was stuck in a javascript loop, draining battery. Its hard to say.