trailer of pallets

It's not unusual to see a trailer full of pallets going down the road ahead of you.  What is strange is when you discover that it's not attached to a truck.  That happened in California.  A driver was filming as he approached the trailer and as he passed it, he noticed that the truck was missing.  It was just headed down the road all by itself.  Apparently the truck driver didn't realize that he had lost his trailer at first.  He was seen on the side of the road about a half mile later. video