What triggers the 'ownership' of a single-authored Answer to be transferred to Community Wiki?
Three ways that I'm aware of. You can do it to your own posts by checking the "community wiki" box when answering yourself. Moderators can toggle this on for anyone's posts. And there's also an automatic trigger when there are too many edits to a question (at last check 10 edits would trigger this, but it's possible for SE to modify the thresholds and algorithms without me knowing).