Over the years I've seen lots of strange things fall off vehicles:
1) A rear wheel came off a car with at least 4 people inside on the Beltline in Raleigh. I was following and saw the wheel wobbling before it parted company.
2) A 6-wheel dump truck had a wheel bearing failure and the wheels and axle slid halfway out of the axle housing before the truck came to a stop. This happened a half mile from my mom's house.
3) A flat-bed semi hauling a 40,000 pound coil of steel left the terminal in Charlotte without strapping or chaining the load down. It fell onto the Interstate a few miles later, breaking the concrete slab. I guess the driver thought it was too heavy to move on the trailer. I didn't witness this one, only saw it on the TV news.
4) Similar to the OP's post, a semi hauling finished lumber lost its load on Interstate 40 in the Pigeon River Gorge in North Carolina. Unfortunately, that time the load went across the wall between the EB and WB lanes of traffic and landed in traffic. I don't remember how many casualties resulted. I didn't witness this one, but I came upon the aftermath and saw it on the TV news.