Here's the deal. If you select 4WD, the transfer case starts sending power to the front driveshaft, which connects to the front diff, which connects to the front axles. With the hubs locked, those axles can turn the wheels. When they're unlocked, they can't. If you had manual hubs and they were turned "off" that would be exactly what happened. Everything but the wheels would be under power. With auto hubs, they axles turn a bit and cause the hub to engage them and therefore the wheels start being powered.
Once you deselect 4WD, the wheels will rotate due to the vehicle movement, which will spin the axles and driveshaft that are no longer connected to the power from the engine. Once they're unlocked, everything between the hub and transfer case is not moving, the front wheels spin just as if there were no axles up there at all.

Brent