I had heard that someone attached a phone to the balloon, but I wasn't over on that side of the field when it happened. But I have found the video on YouTube.
The balloon was a field test by the Airship Victoria. They are building a steampunk dirigible for future exhibition ("To create an audio visual experience within the theme of lighter than air vehicles"), and at the picnic, they inflated this balloon to test its behavior in high winds and near trees. I heard someone say that they actually had expected it to be windier in Golden Gate Park.
Don't get dizzy watching the video.