I'm with the consensus opinion, you were the victim of a Killer Stick.
I've experienced a front tire blowout around 20 mph, maybe faster, coming down a hill. Luckily I was able to control the bike, but that's irrelevant here. While unpleasant, it didn't stop the bike or throw me over the bars. Stopping the wheel suddenly can throw you. A stick that gets caught between the wheel and fork can be responsible for that sudden stop -- and it can break a couple spokes and your valve stem in the process.