In my experience (35 years of architectural practice and currently the IN state building commissioner), spalling like that is usually the result of water evaporating from the finished surface before hydration is complete, weakening the finish surface of the pour. That's why its important to keep the surface damp, especially in warmer weather. Sometimes you'll see visqueen or other coverings used on slabs as they cure, to avoid this kind of thing.
If you happen to get a sizable piece of aggregate near the surface where the water has evaporated, the spalling will take the form of a deeper pit, where the aggregate piece becomes exposed and can work loose, leaving an even bigger hole. As these are exposed to freeze/thaw cycles the concrete will break up further and the damage will grow.
Patching tends to be a short-term fix. You can clean and scarify the surface and pour a new topping slab, but even that isn't a good long-term solution, especially since it would represent a pretty good percentage of the cost of a whole new slab.
