Dead frogs

I've removed four dead frogs fom my pond in the last few days and there is one I can't remove because it is covered in newly hatched tadpoles. No doubt there are others and I can see two that are very weak. No doubt the effort of spawning has taken it out of them but this is the first time I've seen this many. I looked up the lifespan for the common frog and different sites give from 7 to 16 years. I'm just hoping it isn't the disease that is affecting our frog population.