I have tried to determine what causes this because it has happened to me occasionally. It could be the butter was added when the mixture was too warm, or in my case because I make Italian meringue buttercream (same texture as SMBC but cooked differently) I have poured the hot syrup into the beaten egg whites too fast. It needs to be poured in a steady stream while the mixer is on low speed but not too fast or too much at one time. The butter can be too warm when added, or added too much butter at one time. I bring my cold butter just to a room temperature but not soft, soft butter seems to turn to liquid when added. These are the things I have experienced, doesn’t mean it will happen to everyone or that it happens often. Despite any of these problems it can be saved by chilling, bringing to room temp. and re-beaten. I have even frozen it and allowed to thaw in the fridge, brought out to room temp. and beaten and it works perfectly. For any of these issues it is still the best tasting, smoothest buttercream around, I just could never go back to powdered sugar based icing again, too greasy and too sweet!