I've heard many people much more knowledgeable than me state that FTL phenomena/communication would violate causality, but I've never understood why that is or

I've heard many people much more knowledgeable than me state that FTL phenomena/communication would violate causality, but I've never understood why that is or why that violation should be fundamentally impossible. Simultaneity itself is demonstrably relative and it doesn't seem, to my layman's brain, that causality couldn't also be in some form relative.