It is a common observation that when oil and water are poured into the same container, they separate into two phases or layers, because they are immiscible.  In general, aqueous (or water-based) solutions, being polar, are immiscible with non-polar organic solvents (cooking oil, chloroform, toluene, hexane etc.) and form a two-phase system.  However, in an ABS, both immiscible components are water-based.
The formation of the distinct phases is affected by the pH, temperature and ionic strength of the two components, and separation occurs when the amount of a polymer present exceeds a certain limiting concentration (which is determined by the above factors).