Thanks for providing a logical answer to this mystery. It's the only answer that makes sense
“This happened when the severe weather cells were passing through the area.”
I used to be a serious hang glider pilot. At a meet in Italy... several hang gliders were sucked into a giant cumulonimbus cloud that came through the area. The pilots were sucked up to 60,000 ft, suffocated, charred by lightning, frozen and deposited on the ground approximately 60 miles away. This type of accident has caused the deaths of numerous hang glider and paraglider pilots over the years. I am sure that even birds who obviously have better flying instincts than people have suffered the same fate on occassion.