too much sand and water came to the top. Bad curing process.
You see this allot with older driveways, the top layer comes off. This is usually due to the light sand and water sloughing up on the top layer. This layer makes the work look smooth and the heavy aggregate drops to the bottom.
I don't know how he intends to fix this problem now. If he suggests putting pad and carpet over the area then its just hidden and there. But if its in a place where there is vinyl tile there is a problem.