What was Boomer Thinking?

We've had a scary incident.  David had to go to The Big City for a meeting today, so he made an early morning trip to the barn.  He took a load of trash with him, and put it in the dumpster, which is behind the barn.  Boomer freaked.  He galloped to the end of his run and jumped the 5' gate.  He didn't make it over cleanly.  He hit it on the way over, took a tumble, then cantered around the paddock.  David opened his run gate and Boomer came in while David fed breakfast.  David looked him over and walked him, and Boomer seemed fine.  But a few hours later, Boomer didn't seem his usual perky self.  He looked stiff and uncomfortable, so of course we called Dr. Gary.  Dr. Gary examined him thoroughly, and thinks that he may have hurt a ligament in his withers.  We are treating him with pain medication and have hung his hay and water high so he doesn't have to lower his head to eat.  Dr. Gary also took x-rays of his withers with his portable machine, but because Boomer is such a large, heavy bodied horse, the x-rays were unreadable.  If he's still uncomfortable in a week, he will get x-rays in the clinic with a big powerful machine.

We are just aghast and perplexed over this incident.  Boomer is the most unflappable horse we've ever been around.  What was he thinking?  Why did this happen?  Coyotes?  They were very vocal last night.  But I've seen Boomer calmly grazing 20' from a coyote stalking a ground squirrel.  Deer?  That's the only thing I've ever seen him stirred up over, but the other horses weren't stirred up.  They were looking at Boomer, just as perplexed as we are.  David doing the trash?  David does the trash every day, just not that early in the morning.  And Waste Management comes and empties the dumpsters once a week, with a great thumping that makes me spook, but doesn't bother the horses.  We just don't know.