No way it came off something in use...it looks like it has been in a crusher already......probably fell off a crushed car that was being hauled on a flatbed 18-wheeler.
For the past couple of years, as they haul the cruched cars, I have seen them wrapping them with that orange construction fence material....obviously to keep debris from falling off the cars. Brake drums are only held on by the wheel bolted on....and on crushed cars, they always remove the wheels..therefore, the drum would only be held on by friction of the brake shoes or rust.
I know that when you are trying to get a drum off a car, it is stuck on pretty good.