Would not take a 30mm cannon to knock it down, any small hole in the rocket case would vent gas and it would look like someone took a cutting torch to it. The hole would just keep opening and more propellant burning until it opens all the way up and destroys itself.
Stage 1 went fine, a 90 second burn, it was the beginning of stage 2 that took itself out.
Yes I would love for the scenario of a stealth fighter swooping in knocking it down, that would be awesome. (Thats what we think we pay those Air Force guy for any way)However the technology to put something in space is not as easy as everyone thinks. This was more likely a quality issue in the build up of the rocket.