I can't see why this would happen in a virus definitions update, as it doesn't have the functionality to kick off an on-demand scan.
If it could and it did, I'm sure we would be flooded with posts about it after 120529-0, we aren't seeing that so far yours is the first I have seen. I haven't seen this occurrence on either of my systems, winXP/win7 after auto update 120529-0.
Do you happen to have any scheduled scans set ?