My Partner and I had just arrived at the upper tier of waterpipe bay and were checking the topo of 'Locust'. we became aware of some activity above us and a little way to our left (maybe around 'mean mr mustard'). we briefly moved off to the right to get a better view of the route. at this point we could see someone attempting to begin descending into the bay. somehow they dislodged a large block which caused a fair rockfall down the face of the upper tier and into the trees below. luckily we had moved out of the area. upon topping out of our climb i spoke briefly with the climbers involved but they were vague about what had happened. I am unsure whether the block was part of their abseil anchor setup or had become dislodged another way. one of the pair sustained some cuts/graze to their leg. the pair decided to leave after searching for a hold brush they had dropped in the incident.