whats is windows 8 doing when it reboots without notice then spends 45 minutes doing "cleanup"

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I was in the middle of browsing the TechNet site for a download when suddenly my laptop shut down and rebooted into a maintenance mode where I was locked out, it told me &quotcleaning up&quot do not power off your pc. needless to say I was at its
mercy for as long as it decided to take. I found this little hiccup very annoying and would like to know what windows was doing, I am assuming it might be a scheduled &quotcleanup&quot but please warn a guy before just rebooting and locking up my pc for
45 minutes

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