Why is this bird hanging upside-down?

The Hanging Hummingbird has returned. For the third September in a row, on the same feeder on the same Lutsen shoreline property, a female Ruby-throated Hummingbird is displaying what I consider odd behavior. It hangs upsidedown from one of the perches on a sugar-water feeder here.

Three years ago we watched a feeding hummingbird being attacked vigorously by a second bird. The attacker stabbed its victim and pulled at its feathers with its bill. The attack went on for minutes. Eventually, the attackee hung by its feet from the perch, head down. We assumed it was injured. It soon dropped from the perch and lay on its stomach on the deck floor beneath it, then disappeared. It flew away or rolled off the deck into the weeds; I don't know.

The second year a hummingbird hung upsidedown on that feeder without an observed attack. We saw that behavior only once. This year a female Ruby-throated Hummingbird is hanging again, more than once a day, and two days in a row, Saturday and Sunday.

Sunday morning it hung by one claw on one foot, what looked like a precarious position. I thought perhaps the bird was weak or injured. The hanger returned that evening, hanging for perhaps 30 minutes. It was close to dark when it released its grip and flew away. Monday morning, at 8 a.m., there was the Hanging Hummingbird one more time, firmly gripping the perch with all toes on both feet, eyes open, no problem evident. It hung there while a second hummingbird fed, the feeder returning more than once. That was unusual because for three days prior defense of the feeder as food source for one bird only was the rule. Around 8:15 that morning, the bird was gone. Monday evening it hung again, flying away without apparent problem when approached for photos. It returned to hang a few minutes later.

The bird comes and goes it a natural fashion. It flies as it should. It feeds on occasion. It is not bothered by the other hummingbirds using the feeder. Indeed, they feed from perches beside the hanger.

I have no idea why this behavior is occurring. Maybe we've been watching the same bird for three years. The fight three years ago, a serious attack, explained for me the hang at that time; the bird easily could have been wounded. Last year and this year? No idea. I'm contacting a couple of hummingbird specialists to see if they can offer an explanation.