FTA: "Last year's dead zone was smaller than average"

I'm guessing that means that the oil spill isn't a contributing factor, and it's just the excess rain and subsequent runoff this year.

Hypoxic regions are caused by an influx of various nutrient-limited organisms (hence why fertilizer is bad) which trigger a feeding cycle that destabilizes and ultimately kills off the oxygen-producing organisms in the impacted region. Once they're gone, carnivorous inverts which survive well in low oxygen environments move in and stabilize the environment to its newly-barren state.

I don't know for certain, but I'd guess that the chemical dispersants actually contributed to the reduction of hypoxia, since it introduced a new limiting factor for the organisms which start the cycle. That said, I'm sure dispersants aren't biased in what they poison, so it's important to note that this is probably a positive-looking indicator masking devastatingly negative phenomenon.