The branching timeline/alternate universe thing is something they did on an episode of ST:TNG once. It was the one where Worf was shifting from one universe to another because of some technobabble frequency something or other from Geordi's visor.

At one point, Data explained it as "The results of every decision we make or don't make create an alternate universe and timeline, creating an infinitely branching universe and timelines where all possible outcomes occur in one form or another" - don't remember the exact dialogue, but that was the logic.