Dead Frogs

I have had a small pond in my garden for over 20 years and for the first time yesterday I found two dead frogs floating on the surface.  I have had the odd one die after the breeding season in spring but never over winter.  Is it because of the changes in temp.   Or could it be the disease that frogs have.  The pond had been frozen for  most of January and I made no attemps to make any holes as there is plenty of oxygenating vegetation in the margins coming through the ice.  I can see the fish are still at the bottom.  Any clues as to why they have died?

I'm always look forward to seeing the wonderful green of the leaf buds coming into life you don't see any other time of year and the first swallow around the 15th April.  Spring, Spring Spring.