The reason for thinking that they're acausal, at least with the ordinary view of causality as something going forward in time, is that if they weren't, then it should be possible to send a signal faster than light, which according to Special Relativity is impossible.
Note that one way of looking at virtual particles is to imagine a particle moving forward in time and then going back in time a little bit and then going forward. Then the particle-antipartical creation could be called "causal," but it would be caused by something from the future. Many people do this, and it makes them happy. Of course, it goes against how people think of causality, as something going forward in time.
I think it's kind of pointless to focus on a specific idea, such as "causality," in the hopes that if you could get all of them, you'd somehow eventually prove that the universe is nice and simple in a classical sense. Quantum behavior is such that you always have to give up some desirable intuitive idea, and there is just no way to avoid this. You have some flexibility in picking which intuitive ideas you pick, but you can't keep all of them, and the only way to pretend that you can is to be selectively ignorant.