Was it just the back legs/hips affected? Could be Exercise Induced Collapse, commonly referred to as EIC. It's a genetic disorder. Did the parents have all of their health clearances?
Sounds like it could be EIC. Pretty scary when it happens.
OP, so sorry to hear about your lab having EIC. I'm 99% sure based on what you've said, that this is what is wrong. I have seen it before and my last lab had it. It's scary and seems like heatstroke the first time you see it. EIC is a recessive genetic trait in labs only (so far as I know from past research), there is a genetic test your vet can help you with. You can have pups in the same litter that are just fine, affected, or just carriers. On the positive side, it hits each dog differently and you can avoid it happening when you learn the triggers. My last lab had it from both parents (affected) and was only good for a couple retrieves on land before he started to wobble, but he was happy to retrieve all day on the water and loved hunting ducks. he had a normal lifespan and was a normal active lab unless you got him worked up to retrieve on land.
Best thing you can do is get the test done.