Sabayon gone flat.

I'm making a sabayon with a combination of a sauterne and ameretto. I start with just the egg yolks, sugar and sauterne over a bain marie and whisking constantly I add the ameretto to the sabayon after it has doubled in size and it collapses and I have to whisk for another 4-5 minutes to get it to come back to being thick enough to use. Why did the ameretto cause the sabayon to collapse?