What are mechanisms of binary/multiple star formation in different mass ranges (low, intermediate and high stellar masses)?
I don't have enough reputation to comment...
I think this might help you understand the formation or binary and more stars systems. This of course is not the only possible method but it might explain the systems with big mass differences.
As the initial rotation speed increases (marked in the videos as beta) you will see how the protoplanetary disk breaks up and form more than one protostar.