The fact that this happened on a Friday with a Monday holiday coming up smells extremely fishy to me.
Of course this is completely unfounded, but I have a gut feeling its a hack and we will be hearing more about it in the future. Most electronic heists happen at exactly these times because the banks will not reopen until following Tuesday at which time it can potentially be way too late.
One prominent example that I can think off the top of my head is the Bangladesh Bank Robbery [1], but I am sure there are more example to be found