In the event of a secondary control system failure in the engine, the mission rules stated the entire landing phase of the mission would have to be terminated.
“Unlike the roller-coaster ride of Earth orbit, we are entering a slow-motion domain where time and distance seem to have more meaning than speed.” Mike Collins
It looked like they had a leak from one of the propellant tanks on the Lunar Module. If so, that meant abort!
Duke was startled. Why was it shaking so hard? What in the world was happening? There had to be something wrong with this rocket. He didn’t recall any briefing to expect this violent vibration as they sat on the launch pad.