Can someone clarify this for me? Why does the contract with NASA forbid them to restart the engine unless they have a 99% chance of completing a second burn? I

Can someone clarify this for me? Why does the contract with NASA forbid them to restart the engine unless they have a 99% chance of completing a second burn? Is the problem here that if you start a second burn and have to end it prematurely, you could end up in an orbit intersecting the ISS?