Even though it's something that tubular tires are supposed to avoid, my guess is that it's something like a pinch flat. If you hit something hard, the impact can still tear the inner tube.
Imagine hitting a chunk of asphalt or whatever. It collapses the tire against the rim. A tubular rim is a lot nicer shape than a clincher, but still, that old tube is between a rock and a hard place.
As for why it happens after 1,500 miles or whatever, I think just like everything else, stuff gets old. That's a pretty solid number of miles for a Veloflex Carbon.