Because TMSC said it was a variant of the Wanacry virus which means that it is deliberate. You can not accidentally do something in software and then you end up with a virus that takes down nearly all your machines. Not unless there is something deliberate there.
An employee might have clicked on a file they should not have done and accidentally run the virus. That would account for the misoperation part. But nothing in the story accounts for how the virus got there in the first place.
The fact that the tool that was likely infected came from a unidentified source means it could thus be some nefarious actor or competitor.