Foam would have a LOT less momentum than the rocket itself. once it loses contact with the rocket, the air resistance is going to stop it quite quickly. However the rocket, still going, crashes into it, and it does enough damage to a small bit of tile to start the chain reaction upon reentry. A tornado can drive a plastic straw through a piece of plywood, so don't say the foam couldn't cause some damage when it would impact at much greater than the speed at which tornadoes whip things around.