HQ reacts with Oxygen giving Quinone which reacts with Sodium Sulfite giving HQ-Mono Sulfonate and Hydroxide ion which causes the pH to rise slightly. The explanation here is perhaps an oversimplification, but is for you non-chemists. Mees, Mees and James and Haist all discuss this with chemical symbols and the whole detailed agonizing organic chemistry stuff.
This change though, causes a very tiny increase in activity due to the rise in pH but the loss in HQ is what prevents it from being a huge change. Also, the Oxygen dissolved in the water is used up. In the long run, the developer stabilzes and everything is OK. Usually, I let my D-76 sit for 24 hours before use and this damps out the problem.