I have been doing the Apexis Crystal daily, but when I began questing in Draenor I would have the percentage bar appear in the assault areas (when not on the daily or the campaign quest). In fact I was able to complete multiple in the same day if I ended up in the right area. Even now I can occasionally complete more areas than just the daily I selected.
Is this a bug (which I hope sticks around lol), or is there another mechanism for when you can 'assault' an area, ie. weekly? Or maybe just the first time you enter an area it shows up, then after only if on the daily?
Being able to complete the event while on a campaign quest (as per answer below) makes sense, but have also noticed it when I am not doing a campaign.