there is always the question 'but what caused that'? really? there is "always" this question only if you continue to think about the world in the same mindset.
you don't have to search for an answer in the world through Newtonian glasses. "caused" implies causation. causation implies a before, and an after. first there is the cause, then there is its effects. abstract ideas like before and after are looking at the world from a point of view of LINEAR time.
if you truly study relativity
If you truly study relativity, you'll see the words "causality" and "causal" used. It's not a strictly Newtonian idea. As long as there's no faster-than-light travel, "X happens before Y" is an invariant - it's true in all reference frames.
Only if the two events are time-like separated. That is, there does not exist a frame in which they are simultaneous. If two event are space-like separated, there is one frame in which they are simultaneous. Then there is a frame in which A occurs before B and another frame in which B occurs before A. The two events cannot be causally connected.
If two events are time-like separated, (basically means their spacial separation / time separation is less than speed of light), then their order of occurrence is fixed in all frame. Because in this case, it is in principle possible for A to causally effect B since any signal emitted at event A can travel to B at no faster than the speed of light, the causal relationship between A and B is the same in all frame.
tl;dr If A can cause B in one frame, A can cause B in all frames.