"The winning proxy bidder must have seen the BIN, but decided they could get it cheaper. So they let the BIN expire. Then, having a proxy bidder's mentality, they place a bid higher than the original BIN."
I've always viewed that phenomenon as the equivalent of the proxy bidder's shrug. "Oh, well...my strategy didn't work but I really want this item so here goes." Proxy bidders are notorious for being unable to wait for the next identical item that comes up on eBay. It's part of the "gotta have it now" trait that characterizes the breed.