Check the backs of the leaves for insect activity. The cat toilet issue wouldn't cause leaf spotting like that. In fact, it's highly unlikely to cause any problems whatsoever unless there are dozens of cats.
If you see insects, try a spray of soapy water - 1/2tsp. dish soap to a quart of water in a spray bottle and spray all surfaces of the leaves and the stems.
Did you replace or amend the soil where the dead bushes were growing? It's possible there was a soil issue of some kind that caused the other bushes to die. If no insects show up, maybe take a soil sample to be tested at your County Extension office.
