It's really not a mystery if you take the time to think out or draw out the flow of current both during normal operation and during a ground-fault.
Normally operating currents should only flow on insulated or isolated parts on the load side of the service or SDS. However, main bonding / system bonding must be ensured to allow for an effective fault path during a ground-fault.
Study Article 250 carefully and/or get yourself a copy of a good grounding and bonding book.
If you have a more specific question, feel free to post it and we will try to explain it out better.
