Anybody every had this happen:
We were coming across Oklahoma on our way back to Atlanta from a most excellent vacation in the Black Hills when a trucker comes by and starts honking up a storm and pointing at the trailer.
We pulled off and found that one side of the bumper had broken its welds and was dragging along the ground. The other side was just holding on by a piece of one remaining bolt. Another few minutes and the whole thing would have dropped off.
We hadn't heard or seen a thing. (Truckers are great by the way!)
In the picture above I've already taken the spare tire off, then managed to get the remains of the bumper disconnected. We put it on the roof of the Explorer and found a welding shop.
The guy at the welding shop said that only the top edge of the bumper bracket had been welded to the bumper. As the spare tire bounced up and down it flexed that weld and eventually broke it.
He rewelded it around ALL FOUR SIDES of both brackets so it can't move while we're traveling.
I'd check your bumper welds regularly for cracks. Also, if your's is only welded on one side - take it to a welding shop.
If this had dropped off on the expressway and damaged another vehicle . . .