I'm going to guess, and truly it's a guess, but one based on observation, that chickens go into a trance of sorts while dustbathing, like they do with a broody trance. If so, then the physical-mental bond of doing that activity takes a few minutes to undo if it's interrupted. When they dustbathe, it seems to develop into a focus that appears so intense sometimes! I see it in their expressions, they are feeling / thinking just the physical experience, they're not really "looking" at anything.
P.S. Human pictures for tax, we are in a mirror-image world now!