Watching this kind of behaviour, or dustbaths, is fascinating. Anything which doesn't go directly to territory, home building, breeding, or feeding in birds, always interests me because it seems to fall into the "optional/at-will" category. All kinds of models of mind emerge when you bring this choice to the table. Why now? For how long? To what end? Is this pleasurable? Is this more pleasurable than alternatives?
Crows play, slide down roofs, birds catch wind for the fun of it. I guess jumping in puddles may have a biological basis of need.. gumboots or not.