Why does D0 in the delayed choice quantum eraser have two interference patterns?

In the delayed choice quantum eraser it seems like the path through the lens to D0 should be the same as a standard double slit experiment, where it would visibly display an interference patter if there is no which-path information and a double band when there is. However what you get instead is a blob of photons, even if you were to remove BSa and BSb, removing all path information from all photons. This is because there are two interference patterns at D0 which are opposites and cause the peaks of one to overlay the troughs of the other. But why is there even two if it's the same as a normal double slit? It seems like each photon should have propagated through each slit and interfered with themselves producing one interference pattern. So where does the second one come from? Does the future interactions of the other paths photons cause the interference pattern to somehow split into two?