First, let's start with a working definition of determinism. Determinism is "the doctrine that all events, including human choices and decisions, have sufficient causes. "
For example, the death of Raffles has both human decisions and sufficient cause. "Bulstrode does not follow Lydgate's medical instructions, Raffles dies, and Lydgate is under suspicion until Dorothea clears the matter and his reputation. Lydgate realizes people think the 1,000 pound loan he took from Bulstrode was a bribe to "hush" the murder of Raffles. He accepted a loan from Dorothea to cover the debt and cut all communication with Bulstrode" (enotes).
Determinism is one of the most hotly contested aspects of Eliot's novel. Critic Gerald Bullet, for example, argues that "Determinsim is a form of death because it makes everything, including our own thinking, a mindless mechanism." But others, among them George Levine, argue that "George Eliot was a consistent determinist and that this sense was in no way incongrous with her continous emphasis on moral responsibilty and duty." In other words, actions may have predictible consequences, but we are certainly not powerless nor free from culpability of responsible action.