Close-up encounter with foaming Great Crested Grebe

During my last visit to Hyde Park, which was at the end of May, a Great Crested Grebe was swimming near the edge of the lake. Usually when I see one, it is always in the centre of the water - so this was an opportunity not to be missed and I managed to take some close-up photographs of it.

It is a rather lovely waterbird and very handsome. I love the rust colour on it's head and, of course, it's crest.

It seems to be foaming at the mouth, however.

Does anyone know if this is a usual occurrence or is there something wrong with it?