It's the car's fault..... if it hadn't been there, the sign would've landed flat and could go right back up. That sign was obviously meant to fall flat. It was built that way, not to have an obstruction on the ground break it's fall. If I was the owner of that sign, I'd sue the heck outta that car owner. He's caused uncalled for destruction. I'll bet he didn't even have a parking permit to be under that flanged sign with too any faces on it for the weight and shear load it was sporting around.
Call the lawyers, parking and sign police and let's get rockin'