Dude a cannonball was fired within clear sight of a residential neighborhood. Regardless of what you thought of the news reporting quality, it's pretty indefensible, negligent behavior.
From what I understand it hit a hill and bounced which is how it ended up trashing that area. While vary dangerous it was not traveling all that fast. Basically, an 80mph bullet is not that dangerous, but an 80 mph 100lb softball is.
It seems as if this happened at Camp Parks Military Firing Reservation, which they say is surrounded by foothills.