The upper atmosphere is a mess of violent winds, so it wouldn't be out of the ordinary. Don't forget a weightless balloon is a lot more sensitive to turbulence

The upper atmosphere is a mess of violent winds, so it wouldn't be out of the ordinary. Don't forget a weightless balloon is a lot more sensitive to turbulence than a multi-ton computer-stabilized fighter jet, and those winds can already drop a multi-hundred-ton 747 by tens of feet.