Turning under a nitrogen-fixing crop so soon before planting might cause the symptoms of nitrogen excess, depending upon the amount that was produced, and the level of N that was already present. However, I agree, it's hard to believe that a cover crop alone could create such an excess. With most crops, you would think that any excess N would diminish fairly quickly, as it was consumed... but then, since soybeans also fix their own N, that might explain the symptoms.

I grow a lot of different soybeans in maturity Groups 000 through II (they can go up to X, which is tropical), and most of mine bloom in July or early August. I wonder, are there any signs of flowers yet?