I think people have a difficult time separating randomness from causality, so this gets confused. This is a simple example:
Determinism is a stricter form of a causal system. There are parametric systems that are causal relations, but not deterministic.
Just consider a map from some system A to some set of states S = {A1, A2, A3}. This is a completely well defined map, the states A1, A2, or A3 are caused by the system A (it is causal), but there does not exist a one-to-one map from the system A to the set of states S.
Thus it is trivial to imagine a universe that evolves along a parameter t of which the next state of a system S is not deterministically defined given the input (x,y,z,t). One input maps to multiple output states, it is not a functional universe but a relational one, which is a weaker form of causation than determinism. This universe is causal but not deterministic.
Now whether the universe actually IS this way is a different question, but there is no reason why in principle it CANT be this way, is the point. There are causal but non-deterministic systems we can consider.
