Arching is something like an architectural term, but any analogous feature could be said to be arching, it is about something curving. For example, someone arching their back.

A fishing rod could arch or be arching, when a fish is on the line. And the blue skies could be arching overhead, or there could be dismal clouds arching overhead. A dome arches overhead. It is from the analogy of the sky to a dome, that the usage of sky arching overhead derives, I think. Tree branches could be said to arch overhead, over a woodland path. A cat arching its back and hissing.