Why it happens
Cream is an emulsion of milk fat in water, held together by milk proteins. If the sauce boils hard, the proteins clump together and the fat separates out. Acids such as wine, lemon juice or tomatoes make this happen more easily. Lower-fat creams such as single cream, and yoghurt and crème fraîche with less fat, have less fat to protect the proteins and split much more readily than double cream.