I believe two things are happening:
1. You are working with a company regularly, and request a shop. You get that shop and a piggyback shop of the same type. This is sometimes accidental. This has happened to me once or twice. I emailed the scheduler, giving benefit of doubt. Shop removed, no problem,
2. I do think that sometimes schedulers assign workhorse shoppers to stuff they need done. I’m not condoning the behavior at all, but I can see the boards and emails and infer that scheduling can be extremely stressful, so I feel empathy for the stress and frustration they feel. I think they think the shopper will just assume they applied for it. This is obviously not okay.