This is a question I’m sure you have pondered for years, well maybe not. If you are a budding artist or game designer then Simon Schreibt’s series on game art tricks is definitely worth checking out.
Simon has looked at Diablo related art tricks in the past including the resource bubbles, the spider webs, and now angel wings.
In the video below he describes how the movement and look of the wings are likely created. It’s a really interesting watch even if you are not a developer or artist.