I have noticed this behaviour only happens in these following specific cases:
- Your IP has changed several times within the last hour of you being logged in (two or more, in my experience).
- If you have answered more than (three?) quesitons within a five to ten minute period.
- If your answers or questions within this five to ten minute time are "spam" catchable (ie: trips a filter)
- If you have made many (small?) edits to a lot of questions, answers, or comments in a short time.
I am not a StackOverflow.SE or SE developer, so of course I don't know what actually triggers this; but these are the things that make it happen to me. I don't care really, Aurora 10a2 automatically completes Captchas (which is another problem for another day.)