I'm still using xp, I left my computer on this morning and went upstairs when I came back the computer had just restarted and there was a message from microsoft saying an important security update had just been installed 
One of the updates the MS pushed through near the end of support for XP has a bug in it, in that regardless of whether or not it does completely install or not, it thinks it hasn't, so keeps popping up and keeps wanting to install. So that if you leave you PC unattended for a while it will run it and restart.
If I was more cynical than I am

I would think that they did this on purpose to persuade those who have decided to stay with XP for the time being, that they should upgrade.
As MS are not issuing anymore NEW updates for XP, I have turned off the autoupdates setting, and then once every few weeks, turn it back on briefly to catch any misc updates (like the recent patch for IE8) that do come through.
I am not on my XP pc at the mo, but when next on there, I will post the KB number of the rogue patch, so you can see if this is the one causing the issues.