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The simplest answer is that events that happen further apart in space than in time can be observed in different orders depending on the frame of reference. If an effect can happen before its cause, causality is violated.

The three-reference frame example is the easiest, because you can start with a frame where two events, A and B, happen simulateously. A reference frame (say, a spaceship), flying along a line in the A to B direction will observe A happen, then B. A ship flying the opposite direction will experience the opposite, B then A.

So whose observations were correct? All them are perfectly valid. The problem is if we allow A to cause B, in which case the B then A frame has the effect happen before the cause.

>>OkayPh+(OP)
Isn’t cause and effect mediated by forces that travel at the speed of light? Simultaneous events cannot be in a causal relationship, no?