I have heard it discussed in the wvw forums, and it just happened to me. I did damage to the lord and stood in the middle of the circle the entire time. The event did not show up on the side of the screen. It’s a little bit of a let down because it’s a pretty decent reward, and I’m sure there are many people who are missing it.
As far as I’m aware, this only happens in the western keep. I’m not sure if it is specific to a certain borderland.