Multiverse collapse (Time Zero)
A multiverse collapse was instigated by the actions of the Eighth Doctor and Sabbath Dei which temporarily forced all quantum possibilities of history in the post-War universe to collapse into a single timeline. Although intended by the Council of Eight to be a quick procedure, the process was messy and required both Sabbath and the Doctor fight over and sacrifice universes. (PROSE: Time Zero, Timeless) In the end, the Council of Eight's universe was the possibility which crystalized into the only existing reality, paradoxically granting the Council its existence to influence the universe and thus cause the collapse in the first place. (PROSE: Timeless, Sometime Never...)
The specific event which started the collapse was an interruption of a time loop involving Fitz Kreiner's journal An Account of an Expedition to Siberia. (PROSE: Time Zero)
