How does this "spoiler" condition appear? There may be any number of variables involved. I'll identify eight common situations.
1. It may be due to a new (green) slab that has not speedily discharged the excess free water that's essential for blending and pouring. In this case, the culprit may be a 3-inch layer of sand that was placed over a vapor retarder. The combination of the sand and retarder may be preventing water loss at the bottom of the slab, thus forcing most if not all of the free water through the top surface of the concrete.