WP is automatically updating itself to the newest version - what?

I just received 3 emails from 3 different clients' WP sites that the WP has automatically updated to WP 3.8.2 - how is this possible? I have had a couple of these sites for over a year and this has never happened before...I have always gone in and told the WP Admin to update to the newest version manually...All 3 sites were previously updated to WP 3.8 and I don't know if there is a setting somewhere that I need to change to prevent them from updating themselves without me telling them to do so. Any advice/guidance would be much appreciated! Thanks!