Sweeping generalisation - most clubs don't lose supporters since most people don't switch clubs. They just stop going to games for whatever reasons.
There are still 20k+ 'fans' for most clubs in the aleague (as opposed to members or ever been members) and if the football is good, something they didn't like changes, or some other factor that stopped them going to games changes, they may simply choose to come to games.
End of school holidays may be enough in itself for crowds to be boosted. Back to school and work = need to relax or be distracted.
I think the spread of decent football across the league and the chance for most teams to be in the top 6 within a couple of good games is enough to produce crowd growth over the season.
Sydney derby has its own following of course.