If the event that resulted in the Big Bang could occur at one moment, then it could occur the memento before that. And if it could occur at that moment, then it could occur the moment before that. So, the event that resulted in the universe must have always been occurring. That event is and was ongoing.  But the Big Bang has a finite time frame. An event that has always been is in a time frame that has no beginning. The event that resulted in the Big Bang must go from being in an infinite to a finite time frame.

A fluctuation of space can do so because, while the event of fluctuation remains unchanged, change occurs when fluctuations incidentally phase and gravitate toward each other. The change in relative position defines a finite time frame.