Causes of Sudden Death in Dogs?

My six and a half month old Great Dane passed on the 21st. He was fine and showed no signs of anything being wrong with him. I was in my bedroom just getting up and ready for the day and my brother and sister were letting him and our other dog back inside after letting them out to go potty.

They started screaming for me and saying that he was making unnatural sounds, my first thought was that he probably stepped on something and that he'd be okay, but I still rushed to my feet and went over to him. I heard the howl, it was like a human shrieking from pain and when I finally got to him he was lying on his side on the back porch, he was peeing and then it sat there in a pool. I saw his body quiver and he stopped breathing, his tongue fell out of his mouth and his eyes stared blankly off in front of him.

I looked in his throat to see if something was choking him, but there was nothing. He had no bites on him, no wounds of any sort. He just, dropped dead. We don't know why and he is currently at the vet where they are searching for the possible cause of his death. I just want an answer. How does my puppy just suddenly die out of nowhere? I feel like I could have handled this grief less intensely if I had not seen it all play out in front of me. His dying breath and all of the in between will not stop playing behind my eyes. I can not talk to anybody about this because I start to cry at the thought because I can see it all playing out again. The few I have spoken to just say sorry that my dog died. But it wasn't just my dog dying, it was watching it happen and not being able to do anything to save him. Does anybody know what could have caused this? And any advice on how to move forward?