It most likely comes in on a bird from another flock, however healthy it appears, or in hatching eggs from an infected flock. Or having a near neighbor with sick birds.
Twice over thirty years I've seen a sick songbird at one of my wild bird feeders! Emergency! All feeders brought inside...
I'm so sorry for your birds, and that you are having to deal with this. It's why being paranoid about biosecurity is a good thing!
Fortunately it hasn't happened here, and if it arrives, I'll cry a lot and depopulate my flock too, rather than having such a contagious illness present forever...