I think that Jack is going to "screw" things up again with the bomb. Much like he created Evil Ben by refusing the treat him (thinking he could change history) he's going to set off the bomb thinking he can change history again when in fact he'll just set off the accident thus completing the circle of time.
This is why they had to go back. To keep the events of time flowing normally.
This is why they had to go back. To keep the events of time flowing normally.