Violating causality, in this context, is having future events affect past events. This allows a vicious causal circle.
Suppose we have ansibles (Ursula K. LeGuin's instantaneous communicators), and that you're in a spaceship moving very fast relative to me, so that one hour of my time looks like half an hour of yours, from my point of view. Assume also that special relativity holds, so we have no preferred reference frame, and each of us works in our own.
You pass very close to me at one point, and we synchronize clocks and calendars. Two years passes.
At that time, I transmit who won the World Series to you, with a request to copy it back. Two years has passed, so from my point of view one year has passed for you. You receive this when you're one year from our rendezvous, and retransmit it. From your point of view, only six months have passed for me, so I get the copy of the original message eighteen months before I send it, in plenty of time to place a bet before the baseball season opens.
You can play with other assumptions, but it won't fundamentally change the outcome. If any sort of FTL communication is possible, and special relativity holds, it will always be possible to set up some sort of future-to-past causality.