My guess is it was someone already interviewing with multiple companies and another came along with a sweeter offer. Possibly bigger salary or better geography. Maybe a coveted Oncology offer? They definitely burned a bridge with the Regeneron DM and/or Regional, but I don't blame them if the other offer was so sweet that they just couldn't pass it up for the sake of their family and career.
All I know is that my offer here was pretty damn sweet, the people in my district are cool and I have a lot of confidence in the long term potential of the company.