I think I know what happened.... When I cleaned my pond on Sunday, I completely emptied the filter, including all the gunk on the bottom, and all that went into my 5-gallon bucket. I'll just betcha, some dragonfly eggs had gotten sucked up into the filter, and then hatched there inside, and were living at the bottom, as nymphs. When I emptied the filter into the bucket, they went along. Then, maybe they were ready to hatch into dragonflies, so they came up to the top. Or, maybe when I added water from the hose, they were irritated by the chlorine, and that drove them to the top. Either way, I hope they like the new home I gave them (the nearby lily tubs) and that they transform into adults soon.