It can be done with self bag drops (maybe the low cost airlines where you don't show your passport jntil the gate, and can have a family's BPs on the same phone)
Say you've 4 people on a booking, each planning on checking in a bag.
1 person doesn't turn up, and someone scans that person's BP and checks the bag in under that name instead of, or as well as their own.

Perhaps a paper jam that results in a bag tag for someone else coming out, and it's not checked thoroughly by an agent or customer?