Just for the sake of clarity. Are you arguing that causality is relative to the reference frame of the observer?
In the hope that one works, I will give two accounts on this below, and while they involve different ways in in which causality is relative they are akin (although one has pictures I've prepared from a previous explanation). I would recommend reading the second first, which is why I've put it after the first explanation (which, if read, should be read second).
Explanation 1: I'm going to simplify an account I have found has worked in the past (albeit with a different audience and more importantly with the immediacy of spoken dialogue). Also, because greater distances add drama, we're going to have to refer to spaceships, for which I apologize. Imagine you and a colleague are both sent in your respective spaceships, headed for different planets your are tasked to explore. However, for a long while you are both travelling in the same direction and parallel to one another (for the story's sake, this is because your are ensuring that you can properly synchronize your time measurement devices so that the company you work for will be able, from your reports, to know for any time
t how you and your colleague's work was progressing).
The moment before you and your colleague are about to zoom off in different directions (to the left and right, respectively; heading off roughly as if you had both taken off from the same point and were now traveling along the trajectories sketched out by this shape: V). So you make your final time measurement synchronization and hit the hypersuperspeedy-drive button. The blasts from your engines as well as the angle make the hydrothermonuclear laser-protonic propulsion jets (don't worry, it's clean energy) make the jets hit at the point of intersection on the V shape of your trajectories. From that single spot their emerges a lightwave. As everybody knows, ydrothermonuclear laser-protonic jets that hit emit a supremely strong light signal, so strong that when you both reach your respective planets some few light years away, you both turn and see the light-wave emitted. To both of you, the light-wave appears spherical, centered around an origin.
Here's the problem. It's not centered around the same origin. That is, you both see the light-wave as having been produced (caused) at two different places that could be many thousands of miles apart.
The explanation for this effect of relativity is that, as simultaneity doesn't exist, there were two events (the moments when you saw the light-wave) that were simultaneous
only with respect to your individual reference frames. That is, you both saw two different lights. How, though, is it possible for your rockets to have caused a single light-wave to propagate such that you end up seeing two different light-waves?
Explanation 2: This example is more familiar than one involving spaceships, and more directly related to causality. Plus, it involves the hapless victims of physicists everywhere: Alice and Bob.
Bob is standing parallel to train tracks but his direction of gaze is perpendicular to place above the tracks where a train-car carrying Alice will be passing shortly. At the moment the direction of his gaze is perpendicular to her, lightening strikes the front and back of her train-car:
The lightening creates sparks in the train, creating a light-wave propagating towards the front to the back and another doing the reverse:
NOTE: This is from Bob's perspective. He saw the lightening strike the front and rear of the train-car simultaneously and in the second picture he sees them moving as depicted. Alice does not.
Generally, velocity is additive (I can throw a 100mph fastball, I just need to do it from a speeding vehicle so that the velocity of the vehicle is added to my throw (in fact, I can drop a 100mph fastball if I'm traveling 100mph). This isn't true of light. So Bob, who is stationary relative to the train, sees the train traveling
away from the rear light-wave and
toward the front one, meeting at the same point. He thus concludes he was correct, and lightning struck the front and back at the same time:
For Alice, things are different:
Alice is traveling at rest with respect to the train-car, so the light-wave from the front reaches here before she is even aware of a second lightning bolt. For her, lightning struck the front of the train first, and then the rear. For Alice, this EVENT consisted of one lightning strike hitting the front and then moments later a second one hitting the back, and she concludes (correctly) that the light-wave she saw first was caused by lightning that struck first, and the second light-wave was caused by another and later lightning bolt. This "same" EVENT for Bob consists of two light-waves caused by lightning bolts hitting front and back at the same time, and thus for him this EVENT included the meeting of these light-waves and he correctly deduces that this is the result of simultaneous lightning bolt strikes.
Alice and Bob both correctly deduce a causal chain of effects during this EVENT. However, these two chains are different. Moreover, there is no possible way to reconcile them (no preferred reference frame whence we might say that one is "right" as opposed to the other; if it helps, recall that Bob isn't actually stationary at all, as the Earth isn't, and in fact absolute uniform motion doesn't exist).
Special relativity requires us to relinquish the notion of simultaneity, or the idea that there is ever any time "now" except as defined according to some reference frame. Classical causality simplistically reduces to the requirements of an interaction in some local region of space and time. That is, I might turn because a friend who is a hundred meters away yells my name, but I can't turn because of this yelling until the sound-wave
locally interact with my eardrum. However, once you remove simultaneity it becomes impossible to determine the order of events except as they occur relative to some reference frame.
There is no way to say what "now" is, except from one's reference frame. As causes precede effects in the "classical" model, they require a linear progression: x causes y effect, y causes z effect, etc. However, this assumes we have some reason for saying that some cause preceded an effect, but as the order of events is relative to the observer this isn't necessarily true. Currently, at least as long as we don't get into general relativity, only particular causes and effects can be confused here. Because the speed of light provides (supposedly) a boundary on the speed information can travel, locally everything is bounded in what it can "cause" or what "effects" it can be influenced by via its light-cone. Globally, the "now" of you in a space-ship lightyears away from me might be described by coordinates that make us experience the same "now", but small movements in different directions can cause you to shift hundreds of years into the future relative to my "now" or hundreds of years into the past.
