Narcogen offered some thoughts on the grenade chasing. That was so long ago, I've forgotten what he said. But when I get around to doing a movie series on mad Banshees, maybe I'll expand my article and try to include more speculation.

In regard to the circle flying seen in the final part of the movie, I'm sure that was merely a consequence of trying to get to the sniper. The Banshee just happened to approach at a particular angle which caused it to get into an orbit, continually trying to turn in towards the sniper. It's quite easy to get a mad Banshee orbiting you like that.