They periodically check for new updates. You probably just happened to have a periodic check occur shortly after the update came out, just by pure chance. Everybody should see that prompt eventually, but it may take a day or two.
A day or two for an important security update is too long.
I heard about this thing yesterday on HN and did the update manually. My dad and sister's iOS devices hadn't notified them yet when I told them about it today, more than 24 hours later.
