If Fox leads with AFL more often then not it is because it rates lower, not the opposite.
The same reason Coles put the milk at the back of the shop, they know if you are heading in to buy milk you will walk past everything else on the way, and its not like you will step in the door and see the veges and turn around and leave.
Fox sports news has no real-time competition hence why it will run the less desirable stories first, making you wait to see the news you really want to see. How many times do we see a RL headline and not hear the full story till last, after the surfing, tennis, netball and inline skating?
Ch 10 operates on a similar principal since the news runs at the earlier time, but on 9 and 7 they need to run the attention grabbing stories up front to get you hooked or else you might see the AFL and flick to the other channel.