That sounds scary. Seems like it could have flipped up under you and caused you to veer off your lane.
I was following a work truck when a 6 inch square steel plate that was probably 1/2" thick fell off the back of it. It hit the road on a corner and started spinning like a ninja star toward the front of my truck. So fast there really was no time to react but luckily the last bounce it did only put it even with the bumper. It knocked a hole in the bumper and tore off the Toyota emblem above that on the radiator grill then fell off under the truck.
Had it been at the point of the bounce where it was two feet higher it would have gone through the windshield. Guy driving the work truck kept on going, probably never knew one of the 50 plates he had unsecured had fallen off.