As Tryndamere with a Guardian Angel. It is possible to proc your ultimate whilst awaiting to be animated. A bug that can be somewhat detrimental to newer players of Tryndamere, or particularly to players that are awaiting that crucial refresh for tryndamere to get up from ga, and need an immediate ultimate. Right now, I've never NOT been able to activate Tryndameres ultimate whilst awaiting ressurrection from Guardian Angel.
This is a bug, I'm assuming... Since your character is technically, out of action.