I often hear arguments that go like this but I don't know how to respond to them
I notice that your question hasn't been answered yet, but it has been waiting for one for a while. I feel a little out of my depth here, but just to provoke some further response(s). I'll take a shot.
Your question is really about what it means to say that something causes something else. If we think of causation as "deterministic," then the laws of causality work in such a way as to have it be that a given cause will always produce a given effect, and the appearance of this effect can be wholly explained in terms of that cause. There are some scientific fields that seem to work this way--classical mechanics, for example. But then, even in the field of physics, it seems that at some levels this (deterministic) conception of causation seems not to apply--for example, within quantum mechanics. Those who explain what is called the "indeterminacy" at this level might explain it in different ways, but at least in terms of explaining and predicting, it looks like the simplicity of a deterministic model of causation is problematized here.
But if you look at most sciences outside of physics, the very notion that causal explanations are deterministic does not apply. Consider a widely accepted and thoroughly scientifically examined example: smoking causes cancer. Does this causal claim imply that either absolutely everyone who ever smokes gets cancer, or else the claim is false? Of course it doesn't! The same lack of complete determinism may be observed in most of medicine--but that does not prove that "medical science" is somehow a misnomer.
But now extend that to the cases you are asking about. The explanation of the prevalence of PTSD among soldiers and former soldiers is very obviously to be explained by their exposure to the horrible things that they experienced in war zones. Does this causal explanation require that everyone who is in a war zone will get PTSD? Of course not--no more than the idea that every smoker will get cancer (or everyone who falls out of third-story window will die, or everyone who drives when drunk will get in an accident, or ... you get the point). Those who seek to undermine our concerns about bullying or sex abuse with "arguments" like the ones you have mentioned simply do not understand the very nature of causal explanation in the biological or social sciences--and probably don't even understand causal explanation, period (since, as I say, even in some areas of physics deterministic assumptions seem not to apply).
I hope this helps! (Others...?)