Partially "slamming" where agents would try to get folks to switch to a different billing provider, and get paid a commission. So some fraudulently "sign up" random addresses they were supposed to visit.

The other case has been miscommunication over phone or email to someone actually requesting to change billing provider. Or error on the part of the potential customer.

I've had a bill from some random billing provider. In my case it is common for folks newly arriving in the block of flats to get the digits of the address transposed. Due to them using the common English convention, whereas the part of Scotland I'm in uses a different convention.