Presumably paranormal power and/or paranormal strength for the strap. Definitely paranormal levitation.
Why it happens is a good question. Presumably the invading demon is displaying its power, again presumably to impress, dominate and terrify the witnesses. Paranormal stuff purportedly happens in all three stages of possession – infestation, obsession, possession. Perhaps it is all deliberate action on the demon’s part. Maybe some of it is just something that happens as part of the demon’s nature.
Anybody else have suggestions?