I'd guess that probably the store itself is picking out some of the more valuable items from pallets they source, to divert to eBay or Amazon.
This is one of the things that ruined a certain charity thrift store chain for me, which I'd visit often as a destination of daily walks. Although I knew how to spot some valuables (sterling silver, some electronics, some games), they'd never appear. It turns out that the chain does two things that work against brick&mortar hunters: (1) has staff trained to spot valuable items, and divert them to the chain's own eBay store; (2) the chain's distribution/sorting center lets in professional flippers there, to pick out, say, the valuable designer clothing. So the brick&mortar stores only ever get picked-over stuff that wasn't worth either professional group selling elsewhere.
If bin stores are also doing something similar, I'd guess that the smart ones are consciously sprinkling a token number of more-valuable items in the bins (even if they could make more diverted), to keep the lines of hundreds of people forming every week.