My cat died while i was away for the weekend. How did he die?

My husband and I decided to go away for the 3 day weekend. When we got home we found our cat Fez on the couch, in his usual position, sitting on his side but he had blood coming from his nose, and he was dead. He almost looked alive because his eyes were half open and he was reclining in his usual spot and position. We have 3 other cats that were home when it happened and they are being very vocal and clingy. I am just wondering how he died. He was healthy and vibrant when we left...