My guess is that something containing sugar found its way into the concrete, either at the ready-mix plant or on the job. It's anyone's guess how it got there, but there is a strong likelihood that it happened on the jobsite.
We've heard stories about finishers who accidentally spilled coffee (with sugar in it) or soda pop (regular, not diet), fruit juice, or even a piece of a candy bar onto the concrete surface while they were finishing. It only takes that much sugar to retard a small amount of concrete for a long time.
There may be other explanations for this, such as an embedded clump of fly ash that temporarily delayed the set. But from the photo and your description, it sounds sugar-related.
The dark area around the center hole indicates gross retardation. The larger white area probably resulted from efflorescence caused by the continuous bleed of calcium hydroxide to the surface. You probably will have to remove all of the retarded concrete area and patch it.