I'd have thought this would be obvious to anyone posting on Microsoft forums.
It will probably be to do with a difference between computers: there will be some common factor which causes problms on some computers. I don't know what it is in this specific case, but probably something to do with either configuration, installed software
or specific hardware/drivers. It won't be anything obvious - to have got to the point of public release without being identified as a problem is likely to mean it's something obscure.
Updates which only cause problems in certain circumstances are far from uncommon - not just with Windows, but with any software. Those working in IT will come across them from time-to-time but they don't usually get reported other than on specific IT forums.
The only difference here is that this particular problem has affected quite a lot of people, and has a particularly annoying impact - meaning that it's received a lot more coverage than this sort of thijng usually does.