I always put a fly/tarp over my tent/bivy when it's raining. I've never experinced this, but I'd had enough wonderful little drippy spots in the middle of the night that I know I'll break in about 20 minutes of Chinese water torture.

I'm also going to guess that you had a half decent wind at the long axis of your tent? Bernoulli, Venturi, whichever effect that makes airplanes airplanes- IIRC, it should form a vacuum at the short ends if there was. That might have had something to do in your case. But in all honest, Ben, you're the first person I've ever heard of having this happen- Murphy in action.