Hello and thanks for trusting me to help you today. I am a licensed veterinarian with over 30 years of experience and would be happy work with you.
I am very sorry to hear of your loss. It is never easy to lose a pet, but when it is sudden, as you describe, it is often more difficult. There is no doubt that it sounds like he suffered from degenerative joint disease of his hind legs based on your description, but that does not lead to death. It can contribute to discomfort however and some of the labored breathing may have been from that. Most likely, based on the signs you have given, I would be most suspicious of a ruptured abdominal mass, most likely liver or spleen and the weakness and breathing changes that you saw were due to internal bleeding. The liver and the spleen are very vascular organs, so if there is a tumor there and the capsule of the spleen or liver ruptures as the tumor grows to a larger size, it can lead to internal bleeding that can then lead to loss of life rather quickly. This is not an uncommon event in an older, larger breed dog, even if there were no symptoms that lead up to it before. Masses in the abdomen can grow to significantly larger sizes and no one would be aware they were there unless they were specifically looking for them during an exam or imaging procedure like xray or ultrasound.
Please accept my condolences on the loss of you pet.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.