When you upgrade your system, what do you do with the old parts? Individuals may just throw them away or store it up until they decide to throw them away some time later. On the other hand, large companies may have dozens or even hundreds of old system lying in the storeroom. What would you do with the old systems if you were one of the staffs in the IT department?
This may not be the longest or the most sophisticated domino effect ever done but it is still one expensive domino effect. If you ever want to try something like this yourself, just make sure you do not use new harddisks.