From what I have picked up from reading stuff on here for the past year or so ......... it's not common place, but it's also not that unusual.
It can happen for any number of reasons such as pressures of work and family. Also, not everyone is cut out for that method of study and it can be a lonely process at times, requiring plenty of self discipline.
I'm afraid I can't comment on switching from DL to fulltime classroom, but I'm half way through my exams DL and I can back up that what people say about Bristol GS getting results.
Good luck.