What causes antifreeze to overflow into oil on a 1999 Yukon?

Oil does not overflow into the oil. If antifreeze is present in the oil, you either have a cracked head or a blown head gasket. This allows the antifreeze to get into the oil passages and is extremely serious. Stop driving this vehicle immediately. Otherwise you will ruin this engine. The heads will have to be removed and checked for cracks or warping. New head gaskets will have to be installed. Very expensive repair. If you have to travel over a couple of miles to get it repaired, change the oil and filter before taking it in for repair.