I would agree that in many cases it is events that cause probabilities in that the macro world seems to be deterministic in many ways. We often define the toss of the dice as random, but if we were able to measure all of the starting conditions with enough precision we could predict the outcome of every roll as the dice left the hand. In these cases one could argue that randomness is equivalent to ignorance.
However, the quantum world gets a bit trickier. For example, there are no known events that cause one atomic nucleus to decay and not another. Could it be that we are ignorant of some deeper force or process that would allow us to predict which atoms will decay next? I guess that is possible, but from all appearances this type of randomness appears to be an inherent property of the universe.
I will gladly point out that I have not read up on Epicurean philosophy, so I apologize if my ideas were addressed in other posts. These are just my immediate thoughts as it relates to this topic.