Incompetance
I've been saying this for over 30 years.
Even most user's computer infections are relying on the user's lack of computer expertise (not disabling Autorun, unwanted services, adding toolbars, not disabling remote content in email viewer, clicking on OK boxes without reading them, opening unexpected documents to see what they are, not hovering to check links etc etc).
Most really bad IT disaster I've seen have been human error. Even HW failures were everything was lost is human error in sense of not having a backup, RAID or Cluster depending on importance of system. Once there was a server moved while running. Two reasons everything lost. 1) The HDDs only had one or two screws. 2) You don't move stuff that's not portable while running. It's not even a good idea to move a laptop with a regular HDD while running, Dropping it is more likely to be fatal to HDD than when off or asleep.
I even wrote a book about an "apocalypse" caused by human error. Faulty patches to BGP on Routers and on HTPP and eMail on servers on same late Friday.
Biting the hand that feeds IT
