Why and how did my cat die?! :(

Today I found my cat passed away lying on the grass on it's back. It's extremely sad to see him go, but at least I was partially prepared for it - he died almost 16 years old, and with age he was progressively getting weaker and weaker. He was still going outside so it's not like he was sick or anything like that, but for a last month or so he wasn't running (only slowly walking) and wasn't very active outside anymore. His back legs were somewhat weak (several years ago he jumped from my grandfathers balcony and I guess one of his back legs never recovered perfectly) for quite a bit, so walking down the stairs he was always "jumping" with his back legs. Also, he was very skinny and didn't eat a lot. When I found my cat lying passed away, I thought it's a natural death, but then I found some blood (his fur was wet but it wasn't much) on his side at the back and one of his back legs was very still, while the other wasn't. Is it possible that he twisted-broke his weaker leg and this ultimately lead to his death? But how can it cause external bleeding and/or lead to death? 
And then again, if he was attacked by a dog, I would expect much more blood and at least some scars. Another possibility is that he was hit by a car and carried over on the grass - this one is extremely unlikely as he was right next to my PRIVATE, so as you might guess hardly anyone drives here besides me and my family during a day. All the answers are much appreciated. 

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Can't seem to find how to edit my post, but I meant his leg was stiff, not "still".

You will be able to edit your posts once you have been here a little longer. Welcome. I'm so sorry that you came to this site under such sad circumstances. I can't offer you an explanation for death, but I can offer you a forum we go to when our beloved pets cross the rainbow bridge:
http://www.thecatsite.com/f/19/crossing-the-bridge.

We will be here for you.

Weight loss and lack of appetite can be many things. One would be kidney disease, which is something you watch for in cats as they age. There could have also been a heart issue and he could have thrown a clot or his heart may have simply stopped working.

Honestly, without a necropsy (animal autopsy) there isn't really any way to determine cause of death. And, even then, sometimes there isn't a definitive answer.

When was the last time your cat was seen by a vet? Was bloodwork done? That may give you an idea as to cause of death.