I guess chemical warfare of a sort ... I have no way of knowing exactly what. My best guess is that as a last resort of defense, the anemone will release an explosion of nematocysts into the water column, in a closed system like an aquarium they can't be diluted to the point of harmlessness, and they gum up the gills of the fish, and they suffocate to death. The first time it happened, I attributed it to the stress of a tank move. The second time it happened, I attributed it to the stress of a tank move on top of a house move (I just thought I didn't do the move well enough). After the third time, I knew something was up, I had been moving tanks all the time, never losing fish unless this anemone was involved. The fourth time, last December, I thought I had the problem licked, I would move the anemone into its new tank, and have no fish in the new tank, surely that would do it. But within a half hour of the anemone being removed from the tank it was coming out of, my wrasse was dead, and within a few hours after that my cardinal was gone. All I had left was my pair of ocellaris. Who, incidentally, survived every incident prior. (So it was always the non-symbionts who perished.)
I too have heard the stories of massive fish death but it usually involves an injury (fatal or not) to the anemone. In my case there was no injury, just handling. Even when not trying to disturb it off the rock, but taking the rock with it. But mind you it's an extreme challenge to move a 24" anemone and any rock it's attached to.
In fact, my ritteri has been, otherwise, a more or less model citizen. When you create the right conditions for the individual, you should not have difficulties with the anemone moving. A trick that I've found that works well, is to have a coral bommie (aka "a pile of rock") directly under a halide. As long as the currents are nice and strong and alternating at the top of the bommie, and the anemone can not sense a path upwards (i.e., sufficiently far away from the tank walls), it should really stay put quite well.
But my eyes have been opened as to how formidable they can be if disturbed.
(Apologies for the hijack BTW.)