Exactly what happened within those short few pivotal seconds next would depend entirely on whom you ask. Most people watching from padded-leather seats, would tell you it must have been a hefty breeze that went and did it.
If you ask the archers stationed atop their tiled rooftops, and they'd probably tell you it must have been a weak link, an odd-angled odd one out that toppled and crumbled the bunch.
I'm sure Bob, through mouthfuls of ink, would insist in his claims that there was no possible explanation how his impenetrable defense inexplicably fell forward onto him, and Frederick would undoubtedly be there to back him up on it… once he manages to rub the red from his eyes, that is.
And if someone were to ask me for my account, I'd just shrug, I'd just smile, and simply say, "Magic."