I'm also really curious as to how this started. The article mentions a "bug" in the IT systems - some sort of novel zero day in the software they're using that was exploited remotely? Or is it more likely someone screwed up and ran something without thinking?
Edit: There are reports on twitter that this is impacting X-rays, pagers as well as the phone system. This is ridiculous if true and suggests there have been some major failings when putting this infrastructure in place.