Great Crested Grebe's Nest

Yesterday morning I went to check on the progress of what I think was a second brooding for this pair of Great Crested Grebes. A disaster had struck as far as I could tell, and recently. Instead of one bird incubating on the nest, both birds were behaving as though something was wrong. One bird, I think probably the female, eventually climbed onto the nest to check out what had happened. There weren't any eggs or chicks to be seen anywhere. She then sat on the nest which appeared very wet, but not for long. She then left the nest and before both birds left the area, the remains of the nest sank. I carried these images around in my head all of yesterday, and still don't know what had caused the destruction of the nest. A challenging world for birds...