The irony being, they were covering Bob Dylan's "Rainy Day Women #12 & 35."
In all seriousness, one wonders what lead to this rock-throwing. People don't normally go to concerts with the intention of throwing rocks, so what was the difference in this case?
Was the crowd simply overly rowdy? Was Nickelback part of a larger concert and considered out of place? Did Nickelback somehow offend the crowd? Certainly, the dumb schmucks who threw the rocks should be held accountable, but there may be an explanation for what happened.