the fish not only get stressed, but shocked. At that point they're stunned and probably take little notice of the surroundings, such as the filter, and do little to resist it.
I was in a similar situation once - I've had a longfinned rosy barb once, that was super-agressive compared to how these fish usually are, that nipped and nipped and nipped. He killed 3 fish this way, and almost finished off my favorite Tiger barb, but I isolated the rosy barb soon enough, and the tiger recovered. I got rid of him after about a week of isolating him. And since then, everything is well.
Anyway, I'm just saying that this does occasionally happen, sometimes fish are a bit aggressive within generally peaceful breeds. Get rid of that orange tetra, and get some new rainbows.