Just guessing, no I have no idea why it would sour, unless it would be that the grass clippings and leaf mulch has created a big 50 X 50' active compost pile that the cold, damp weather has let sour. Unless you have some more dry, dead leaves, cotton burrs, or fine hay to shred up and add, I would just pray for some dry, warm weather with wind to dry it out some. That would probably work wonders and you might still get a crop in.
Good luck and keep us posted.
Bill P.

Your soil smells really bad because it is anaerobic, too wet and you should not be tilling that soil yet. Your soils pH of 6.5 in about in the middle of the desireable range of 6.2 to 6.8 but most likely you still do not have enough organic matter in that soil. Wait to till more until you can pick up a handful of soil, squeeze it tightly, and when you poke the ball that is formed it just falls apart. At that time your soil is ready to till, if it is necessary to till.

Watch out with terms- "sour soil" is normally used to refer to acidic soil (and your pH is great).

OK- it smells sour after you tilled in grass clippings and leaves. My guess is about the same as kimmsr- the leaves, and grass are holding lots of moisture and creating anaerobic conditions. The wet soil, and maybe some matting of material, is limiting air to the site of composting. This is not as big an issue as you might imagine- many bury compostables and let them decompose in the bed with great results.

If I was you- I'd stop tilling so much and let it do its thing. When the leaves and grass decompose and the soil dries a bit the soil will breathe a lot better. In the future- just adding that stuff mixed up and left on the surface will add all the goodies without the anaerobic decomp.

reeldoc...just out of curiosity, what color is your soil?