PoD: September 11, 2001
After the first plane hits the North Tower, whoever is in charge at the World Trade Center decides to take no chances and orders the South Tower evacuated immediately, just in case (as opposed to the confusing orders of OTL).
It should be noted that many companies in the South Tower OTL did evacuate employees before the second plane struck the South Tower (some of them went back up due to the aforementioned orders)...
Effects, anyone?