Below are some thoughts I was trying to develop. Everyone is welcome to point out where I'm getting off-base.
1. If determinism is true, there is no modality: everything is necessary. So, if you take modalities like possibility and contingency to be real, determinism is false.
2. If determinism is true, there is no “real” (non-Humean) causality: everything is connected necessarily and symmetrically; the world is fixed forever. Real causality implies modality: things could have happened differently if causation is assumed to involve real “work”. Like modality, real causality is opposed to determinism.
David Lewis would be able to embrace both modality and determinism by postulating concrete possible worlds (which are causally unconnected to ours) as the vehicle for modality. But he is a Humean about causality. There is no real causation in his system.
The only way to embrace modality and real causation is to reject determinism. This implies that the world includes an intrinsic selection or choosing among possibilities within causal events.
An assertion that only objectively random choosing occurs at this point is ad hoc: it has no particular advantage over asserting that choosing is non-random.