Why did my puppy's tooth fall out?

Today when I was in the car, my 7 month old puppy was perfectly fine. Then suddenly he started making gargling noises and was hing at the mouth. When I opened his mouth to see what it might be, there was blood in his mouth! After a few minutes he started to make sounds as if he were crunching on something. I was trying to comfort him and he spat out a tooth. It looked like a back tooth. I will be taking him to the vet, but why did he lose his tooth? What could it be? It didn't look black or anything.