I'm so sorry. That sounds like a terrible day and it's natural that it still haunts you.
Without an autopsy- and maybe even with one- you'll probably never know for sure. That's hard. I share Kimber's story in the hopes that it gives you a little more information. When Kimber was 7, she was in great shape. We competed in agility and went on hikes and long walks with her every day. She got a ton of exercise and just ate it up. She was in perfect health. And then one day we were playing fetch and she just collapsed. I was luckier than you- she didn't die- but she was suddenly completely paralyzed in the back end. The local emergency vets were stumped and referred us to Virginia Tech which has a veterinary school. The neurologist said it was a spinal bruise or spinal stroke (also known as an FCE). Since both have the same prescription (palliative care + rehab) we never pursued a final diagnosis. I'm now pretty sure it was the former but I learned a lot about FCE's. Labrador retrievers are one of the breeds susceptible to FCEs. I'm not sure if they are fatal- I was focused on the rehab literature- but it seems likely if labs can have spinal strokes they may also have other types of strokes.
The board is a little quiet on weekends but I hope others will chime in with their experiences soon.