Weird, I follow Adam Savage on Twitter and he hasn't said a thing about it yet. Maybe he's not allowed to just yet.
Apparently he wasn't there, it was the other trio that were doing it. They were tweeting about it at the time, but they have since removed them all (presumably at the request of lawyers). They are archived in a few places if you can be bothered looking (I could not).
these guys are usually all over the math... im thinking something pretty bad had to have happened like the angle changing at the last minute? or the ball delaying as it exited the canon, the canon changes angle and then the bal was shot at a high angle... further distance... not hitting the barrol etc
Someone on Slashdot theorised that what happened was the cannonball went straight through their safety wall thing, and then there's this hill behind it that serves as a natural barrier. Instead of embedding into the hill though it hit it at such an angle that it glanced off and basically bounced over it or off in some weird direction, which sounds feasible to me.
It sounds like they had a couple of safety precautions but they all failed because they basically underestimated the power of the cannon.