Hi there. This is a known issue being investigated. What we've seen cause this is players trying to hide from attacks around the outer edges and because the Lightbearers aren't blocked from the edges, they try to get to the player and end up going out over the water where they'll fall.
Best thing to do to avoid this issue is try killing the wizard first, and don't hide around the outer edge of the platform for cover. You can stick to the open area and inside the buildings at each corner.
The first time I was hiding like that but this evening no one was around the back edge. We killed her near the back left area and she just kind of floated out of bounds and into the water.
I guess I'll try it again some other time. Thanks for the quick response.