I think in some of these cases the items are pre-packaged for shipment. There's some inflection point where 90% of customers who order item X order only that item, and it makes sense to have them ready to just slap a label on and go. I'd imagine that the packing stage could become a bottleneck for a busy time at a DC, and pre-packing single items would not only lighten the load during a rush, it would also allow those packing stations to remain occupied during a lull.

Obviously, Prime also pushes people toward single-item orders, which helps justify this strategy, and also increases people's dependence on a service level that is hard for competitors to match who aren't at a scale where these kinds of approaches work out.

Kind of a fail that the system wouldn't flag a 20-parcel order as something that needed a human to ok it, but who knows, perhaps it was reviewed after the fact and something got tweaked.