Did you use a metal tool to mix the paste or spread it? It only needs the smallest amount of iron, and the moisture of the glue on the metail can get that, for it to react with the tannin in the oak. Another possibility is if you used water to make the paste more workable, as that may well have iron compounds in it. I had a similar problem when making an oak table with a customer. He insisted on wiping off the excess glue with a damp rag (In my own workshop I let the glue go off then remove any excess with a paring chisel). The result was black staining all along the joint lines.