At the top of the crag, preparing for the abseil down, the involved person was partially resting on their rope clove hitched to the anchor. They were asked by the person setting up the abseil to untie their other rope so it could be threaded through the anchor for the abseil. Instead of untying the unweighted rope, they started to untie the knot attaching their harness to the anchor. They got the majority of the way through untying this knot before it was spotted by someone, and they were able to tie back in. If they had undone another rope strand, it is likely they would've fallen 20m to the floor.