sounds like anacron doing its job....
with a default install, the anacron service is set to run all the cron jobs 65 minutes after you boot your computer. Among other things, it runs stuff like prelink, logwatch, and updatedb which updates the database of files on your computer so you can search with the locate command. Running these jobs causes huge disk activity for a few minutes.
you can turn the anacron service off and rely on cron if you don't want these "interruptions". By default, the cron service will run the jobs at 4:02am. If your computer is off, they won't run at all that day, hence the usefulness of the anacron service.
you can change the time the cron jobs run or the delay factor for anacron if you feel comfortable editing some files as root, /etc/crontab and /etc/anacrontab